Process for making aluminium alloy sheet

Abstract: A sheet of aluminum alloy containing magnesium, silicon and optionally copper, characterized in that the aluminum alloy of the sheet contains amounts in percent by weight of magnesium and silicon falling within the area ABCDEF of Figure 1 of the accompanying drawings, and amounts of copper between the contours shown by broken lines in Figure 1 and 0.3 wt.% in area BHGI and 0 wt.% in areas HAFG and IEDC of Figure 1; and optionally at least one additional element selected from Fe in an amount of 0.4 percent by weight or less, Mn in an amount of 0.4 percent by weight or less, Zn in an amount of 0.3 percent by weight or less; and optionally at least one other element selected from Cr, Ti, Zr and V, the total amount of Cr + Ti + Zr + V not exceeding 0.3 percent by weight of the alloy; the balance being Al; and in that the sheet has been heat treated to have a T4 temper strength, after natural aging and levelling or flattening, in the range 90-175 MPa and a potential T8X temper strength of at least 170 Mpa by a treatment selected from: (a) solution heat treating said sheet at a temperature in the range of 500 to 570°C and then cooling said sheet according to a scheme comprising cooling to between 350°C and 220°C at a rate greater than about 10°C/sec but not more than about 2000°C/sec, then cooling to a temperature in the range of 270°C and 140°C at a rate greater than 1°C/sec but not faster than 50°C/sec, then cooling to between 120°C and 50°C at a rate greater than 5°C/min, but less than 20°C/sec, and then cooling to ambient temperature at a rate of less than about 10°C/hour; (b) solution heat treating said sheet at a temperature in the range or 500 to 570°C and then cooling said sheet according to a scheme comprising cooling to between 350°C and 220°C at a rate greater than about 10°C/sec but not more than about 2000°C/sec, then cooling to a temperature in the range of 270°C and 140°C at a rate greater than 1°C/sec but not faster than 50°C/sec, then cooling to between 120°C and 50°C at a rate greater than 5°C/min, but less than 20°C/sec, coiling said sheet and then cooling to ambient temperature at a rate of less than about 10°C/hour; or (c) solution heat treating said sheet at a temperature in the range of 500 to 570°C and then forced cooling said sheet using a means of cooling selected from water, water mist or forced air, and coiling said sheet at a temperature of between 50 and 100°C, then allowing said coil to cool at a rate of less than about 10°C/hour.

Description: TECHNICAL FIELD This invention relates to continuous processes for making sheet material from aluminum alloys useful, in particular, for automotive applications. More particularly, the invention relates to processes for making sheet from alloys of Al-Mg-Cu-Si and Al-Mg-Si. BACKGROUND ART The automotive industry, in order to reduce the weight of automobiles, has increasingly substituted aluminum alloy panels for steel panels. Lighter weight panels, of course, help to reduce automobile weight, which reduces fuel consumption, but the introduction of aluminum alloy panels creates its own set of needs. To be useful in automobile applications, an aluminum alloy sheet product must possess good forming characteristics in the as-received T4 temper condition, so that it may be bent or shaped as desired without cracking, tearing or wrinkling. At the same time, the alloy panel, after painting and baking, must have sufficient strength to resist dents and withstand other impacts. Several aluminum alloys of the AA (Aluminum Association) 2000 and 6000 series are usually considered for automotive panel applications. The AA6000 series alloys contain magnesium and silicon, both with and without copper but, depending upon the Cu content, may be classified as AA2000 series alloys. These alloys are formable in the T4 temper condition and become stronger after painting and baking. Because thinner and therefore lighter panels are required, significant increases in strength after painting and baking will be needed to meet these requirements. In addition, known processes for making sheet material suitable for automotive panels from the alloys has involved a rather complex and expensive procedure generally involving semi-continuous direct chill (DC) casting of the molten alloy to form an ingot, scalping the ingot by about 6 mm (1/4 inch) per rolling face to improve the surface quality, homogenizing the ingot at a temperature between 500 and 580°C for time periods between 1 to 48 hours and hot and cold rolling to the desired gauge. The rolled material may then be given a solution heat treatment at 500 to 575°C for 5 minutes or less in a continuous heat treatment line, rapidly quenched and naturally aged for 48 hours or more. In this procedure, the scalping and homogenizing steps are particularly troublesome. Moreover, the homogenizing step prevents the sheet from being produced essentially continuously from the casting step to the re-roll step following hot rolling. An aluminum alloy is disclosed in European Patent Application EP-A-282 162 published on September 14, 1988 in the name of Alcan International Limited. The aluminum alloy of the reference has a preferred composition containing Cu, Mn, Si, Fe and Cr, with a reltively high content of Mg (1.5-2.4%), and sheet may be made from the alloy by direct chill casting or twin belt casting, followed by hot and cold rolling, with solution heat treating and quenching before a final cold rolling step. The Examples disclose a process as that of the invention, however without mentioning coiling or disclosing coiling temperature. Moreover this alloy is intended for the manufacture of beverage can ends, and consequently has a high initial tensile strength to withstand the substantial stretching deformations typical of can-end making operation, followed by minimal loss of tensile strength during subsequent lacquering and stoving. Such characteristics are not suitable for the manufacture of autobody panels, where a relatively low tensile strength is required followed by the development of a high tensile strength during painting and baking. US Patent 5,266,130 which issued on November 30, 1993 to Uchida et al. discloses a process for manufacturing aluminum alloy material suitable for the body sheet material of automobiles. The process is carried out on an aluminum alloy containing silicon and magnesium and involves solution heat treatment followed by first and second stage cooling at different cooling rates. Coiling or coiling temperature is not mentioned. The product is said to have excellent shape flexibility and bake hardening. However, the inventors of the present invention do not see this procedure as entirely suitable. There is therefore a need for improved processes for fabricating sheet material from aluminum alloys. DISCLOSURE OF THE INVENTION An object of the present invention is to provide new alloys that facilitate procedures for making alloy sheet material useful, among other purposes, for automotive applications. Another object of the invention is to provide aluminum alloys that can be made into strip by a belt casting procedure, for subsequent conversion to sheet material suitable, in particular, for automotive applications. Another object of the invention is to provide an improved procedure for producing alloy sheet material that avoids the need for scalping of the cast ingot and homongenizing of the alloy. Another object of the invention is to provide an alloy product demonstrating improved strength after a paint bake cure. Another object of the invention is to improve quenching methods to yield stronger aluminum alloys produced by belt casting or other means without sacrificing formability. The invention is defined in claim 1, optional features thereof being comprised in the dependent claims. The invention primarily refers to making an aluminum alloy sheet resulting from a twin belt casting process and a hot and cold rolling process; the aluminum alloy of the sheet contains amounts in percent by weight of magnesium and silicon falling within the area ABCDEF of Figure 1 of the accompanying drawings, and amounts of copper between the contours shown by broken lines in Figure 1 and 0.3 wt.% in area BHGI and 0 wt.% in areas HAFG and IEDC of Figure 1; the alloy is the result of the twin belt casting process carried out with a heat extraction rate within the range defined by the following equations: Lower bound heat flux (MW/m 2 ) = 2.25 + 0.0183 ΔT f Upper bound heat flux (MW/m 2 ) = 2.86 + 0.0222 ΔT f Lower bound of alloy freezing range = 30°C Upper bound of alloy freezing range = 90°C where ΔT f is the freezing range of the alloy expressed in degree Centigrade. The alloys may also contain at least one additional element selected from Fe in an amount of 0.4 percent by weight or less, Mn in an amount of 0.4 percent by weight or less, Zn in an amount of 0.3 percent by weight or less, and a small amount of at least one other element, e.g. Cr, Ti, Zr or V, the total amount of Cr + Ti + Zr + V not exceeding 0.3 percent by weight of the alloy. The sheet is heat treated to have a T4 temper strength, after natural aging and levelling or flattening, in the range 90-175 MPa and a T8X temper strength of at least 170 Mpa by a treatment selected from: (a) solution heat treating the sheet at a temperature in the range of 500 to 570°C and then cooling the sheet according to a scheme comprising cooling to between 350°C and 220°C at a rate greater than about 10°C/sec but not more than about 2000°C/sec, then cooling to a temperature in the range of 270°C and 140°C at a rate greater than 1°C/sec but not faster than 50°C/sec, then cooling to between 120°C and 50°C at a rate greater than 5°C/min, but less than 20°C/sec, coiling the sheet at a temperature of 50-100°C and then cooling to ambient temperature at a rate of less than about 10°C/hour; or (c) solution heat treating the sheet at a temperature in the range of 500 to 570°C and then forced cooling the sheet using a means of cooling selected from water, water mist or forced air, and coiling the sheet at a temperature of between 50 and 100°C, then allowing the coil to cool at a rate of less than about 10°C/hour. The alloy sheet may either be produced by belt casting followed by hot and cold rolling or by conventional means such as direct chill casting followed by scalping, homogenization, hot and cold rolling. The aluminum alloy sheet material is suitable in particular for automotive applications, in which alloy slab is produced in a belt casting machine by casting an alloy of aluminum while extracting heat from the alloy, hot rolling and cold rolling the slab to form a sheet, solution heat treating the sheet to re-dissolve precipitated particles, and cooling the sheet. The sheet preferably exits the forced cooling at a temperature of between 120 and 150°C and the sheet is preferably coiled at a temperature of between 60°C and 85°C. When forced cooling to between 120 and 150°C is employed, the sheet is preferably passed through an accumulator in which it cools further to between 50 and 100°C and preferably 60 to 85°C, prior to coiling at that temperature. The cooling steps which follow the solution heat treatment of this invention may be referred to as a controlled quench process. Reference is made in this disclosure to metal tempers T4 and T8X. The temper referred to as T4 is well known (see for example Aluminum Standards and Data (1984), page 11, published by The Aluminum Association). The alloys of this invention continue to change tensile properties after the heat treatment process and are generally processed through a flattening or levelling process before use. The T4 properties referred to therefore pertain to sheet which has been naturally aged for at least 48 hours after the heat treatment of this invention, and has subsequently been processed through a tension levelling process. This is in keeping with normal commercial practice for this type of alloy. The temper T8X may be less well known and it refers to a T4 temper material that has been deformed in tension by 2% followed by a 20 minute treatment at 170°C or a 30 minute treatment at 177°C to represent the forming plus paint curing treatment typically experienced by automotive panels. Potential T8X temper properties refer to the properties that the material of the given composition, subject to the processing step and thermal treatment will develop in a future process, such as a paint-bake step, that is equivalent to the T8X temper. The above composition limits have been set first by the need to reach the tensile and formability property targets as set out in Table 1 below and, second, by the need to avoid the formation of second phase constituent particles from the primary alloying additions which will not be redissolved on solution heat treatment and which, therefore, do not add to the strength of the material but which, at the same time, will be detrimental to the formability. Thirdly, the composition limits have been set to ensure that the minimum solid solubility temperature range for the major alloying additions is at least 20°C and preferably greater than 40°C to ensure that the material can be effectively solution heat treated in a continuous strip line without approaching the temperature at which liquation and ensuing strip breaks would occur. When the above alloys are produced by belt casting, it is a particular and surprising feature of the invention that it is possible to obtain automotive sheet with the desired T4 and potential T8X properties without the need for homogenization and scalping. It has been discovered that this occurs only if the belt casting is carried out for a specific heat flux extracted by the belts, which is related to the alloy freezing range (ΔT f ), by the requirement that the heat flux lie in the area of heat flux versus alloy freezing range bounded by the following equations: Lower bound heat flux (MW/m 2 ) = 2.25 + 0.0183 ΔT f Upper bound heat flux (MW/m 2 ) = 2.86 + 0.0222 ΔT f Lower bound of alloy freezing range = 30°C Upper bound of alloy freezing range = 90°C where ΔT f is the freezing range of the alloy expressed in degree Centigrade. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a chart showing Mg, Si and optionally Cu contents of aluminum alloys according to the present invention; Fig. 2 is a chart similar to Fig. 1 showing the composition of preferred alloys; Fig. 3 is a chart showing acceptable heat extraction rates for alloys according to the invention of various freezing ranges; Fig. 4 is a chart similar to that of Fig. 1 showing alloy compositions for which a special quenching procedure is particularly preferred; Fig. 5 is a schematic illustration of steps carried out according to a preferred embodiment of a process according to the invention. BEST MODES FOR CARRYING OUT THE INVENTION While the sheet made according to the present invention can be used for other purposes (e.g. canning, building sheet materials, etc.), use for automotive applications, e.g. panels and skins is primarily intended. As such, they sheet should desirably have a relatively low T4 strength (e.g. in the range of 90 to 175 MPa) to allow for easy part forming by automobile manufacturers, but a relatively high eventual T8X strength (e.g. 170 MPa or more, and more preferably 200 MPa or more) developed as a result of a typical automotive painting and baking procedure, in order to provide high resistance to denting. Other properties, such as good corrosion resistance, good surface quality, etc., are also clearly desirable. These desirable properties and others are shown in Table 1 below: Property Values Yield Strength, T4 90-175 MPa Yield Strength, T8X ≥ 170 MPa preferably ≥ 200 MPa Total Elongation, % ≥ 25 Erichsen Cup Height (inches) ≥ 0.33 Bend Radius to Sheet Thickness Ratio, r/t ≤ 1 Plastic Anisotropy, R ≥ 0.60 A T8X of at least 170 MPa gives adequate strength after paint bake for many automotive sheet applications, but for the automobile body sections that are most critical, a higher T8X of at least 200 MPa is generally preferred, and therefore the preferred value of T8X for this invention is one that is at least 200 MPa. It has been found that certain Al-Cu-Mg-Si and Al-Mg-Si alloys of the AA2000 and AA6000 series can not only be fabricated into sheet material having many of the desired characteristics mentioned above, but surprisingly they can be cast by a procedure involving belt casting, such as twin belt casting, without the need for subsequent scalping of the resulting ingot surface and homogenizing of the product. This means that the fabrication of sheet material suitable for automotive applications can be made essentially continuously from caster to re-roll, thus facilitating the manufacturing process. The aluminum alloys which have this advantage are those having compositions falling within the indicated volume on the chart of Figure 1. This volume is defined by boundaries ABCDEF, which circumscribe the permitted silicon and magnesium contents of the alloys, upper contours 10 (shown in broken lines) within the boundaries ABCDEF, which specify the maximum copper contents of the alloys having particular magnesium and silicon contents, and lower surfaces (not shown) within the boundaries ABCDEF specifying the minimum copper content of the alloys at particular magnesium and silicon contents. The lower surface is at a copper content of 0.3 wt.% in Region I (BHGI), at a copper content of 0 wt.% in Region II (HAFG) and a copper content of 0 wt.% in Region III (IEDC). Thus, the effective alloys falling within the defined volume are those having approximately the following Mg, Si and Cu contents in wt.% of the total alloy:- (1) 0.4 ≤ Mg < 0.8, 0.2 ≤ Si < 0.5, 0.3 ≤ Cu ≤ 3.5 (Region I) (2) 0.8 ≤ Mg ≤ 1.4, 0.2 ≤ Si < 0.5, Cu ≤ 2.5 (Region II) (3) 0.4 ≤ Mg ≤ 1.0, 0.5 ≤ Si ≤ 1.4, Cu ≤ 2.0 (Region III). The above ranges are said to be approximate because the maximum values stated for copper are suitable only for certain Mg and Si contents and lower values are suitable for other Mg and Si contents, as shown in Figure 1. The preferred maximum copper concentration for a particular Mg and Si concentration will be that which results in a solid solubility temperature range of at least about 40°C. However, it is noted that a s lid solubility range of at least about 20°C may be acceptable though not preferred. In addition, the alloys may optionally contain Fe ≤ 0.4 wt.%, Mn ≤ 0.4 wt.%, along with small amounts of other elements (e.g. Cr, Ti, Zr and V, such that the total amount of Cr + Ti + Zr + V does not exceed 0.3 wt.%). The balance of the alloys is aluminum and usual or unavoidable impurities. These alloys may also be cast from recycled metal in which case zinc may be found as an impurity because of the pre-treatment applied to the original metal sheet. However, the sheet can still meet all requirements for levels of zinc where Zn < 0.3 wt%. These alloys generally have freezing ranges of 30 to 90°C, which allows them to be belt cast to obtain acceptable surface characteristics and yet at the same time to avoid a significant amount of internal and surface segregation and second phase formation. These properties and T4 and T8X properties needed for automotive sheet require, however, that the belt casting process be carried out within the band of heat fluxes shown in Figure 3. Moreover, the alloys have a solid solubility range of at least about 20°C and more preferably at least about 40°C under typical commercial heat treatment line conditions. For a particular Mg and Si concentration, the preferred maximum amount of Cu is such that for Cu concentrations less than or equal to the preferred maximum the solid solubility temperature range will be at least 40°C under typical commercial solution heat treatment line conditions. The Cu contours in Figure 1 represent this preferred upper limit of copper. This means that significant amounts of Mg, Si and, if present, Cu can be brought into solid solution through a solution heat treatment, rather than forming small range compositional variation type particles. This allows the sheet material to be successfully processed in a typical commercial continuous heat treatment line without causing breaks or the need for conventional homogenization. The compositions of preferred alloys are those previously described (and illustrated in Fig. 1) except that the Mg and Si concentrations are limited to those lying within the shaded area INAFEM of Fig. 2. The alloys having compositions within this volume have the best casting characteristics and optimal final properties. The area INAFEM is bounded by the following equations: Mg = 0.4% (Line IM) Mg = 1.375% - 0.75 x %Si (Line EM) Si = 0.5% (Line EF) Mg = 1.4% (Line AF) Si = 0.2% (Line AN) Mg = 1.567% - 2.333 x %Si (Line IN). The alloys defined in Figs. 1 and 2 may be subjected to belt casting using any conventional belt casting device, e.g. the twin belt caster described in U.S. patent 4,061,177 to Sivilotti, the disclosure of which is incorporated herein by reference. However, the casting may alternatively be carried out using a twin belt caster and casting procedure as disclosed in U.S. patents nos. 5 636 681 and 5 671 800 (EP-A-773 845). This latter device and procedure employs a liquid parting agent (e.g. a mixture of natural and synthetic oils) applied in a thin uniform layer (e.g 20 to 500 µg/cm 2 ) by a precise method (e.g. by using electrostatic spray devices) onto a casting surface of a rotating metal belt prior to casting the molten metal onto the belt, followed by completely removing the parting agent from the casting surface after the casting step and re-applying a fresh parting agent layer before the belt rotates once again to the casting injector. The apparatus also employs a flexible injector held separate from the casting surface by wire mesh spacers which distribute the weight of the injector onto the casting surface without damaging the surface or disturbing the layer of liquid parting agent. The device and procedure make it possible to cast a thin strip of metal on a rotating belt and to obtain a product having extremely good surface properties, which is valuable in the present invention. Whichever type of belt casting procedure is employed, it is important to ensure that heat is extracted from the molten metal at a certain rate during the casting process. If the rate of heat extraction is too low, surface blebs or segregates develop that give rise to unacceptable surface finish. Further, excessive segregation and second phase formation occur within the cast strip such that these cannot be eliminated by subsequent solution treatment within a reasonable combination of time and temperature. On the other hand, when the heat extraction rate is too high, surface distortion may occur during the freezing process. This locally disrupts the heat extraction and hence the freezing process, resulting in regions of coarse second phase particles, porosity and, in severe cases, cracking. It has been found that the above phenomena are correlated to a combination of the freezing range of the alloy being cast, which is dependent upon the composition of the alloy, and the rate of heat extraction (that is, the heat flux through the belts used to contain the cast metal during solidification). The relationship between freezing range and heat extraction rate is shown in Fig. 3, the acceptable heat extraction rates being shown in the shaded band of the graph. Material to the left of the band is too soft, while the material to the right is too strong, and may exhibit large intermetallic and eutectic segregate formation. The solid solubility range for the material to the right of the band is also too short. Material above the band shows shell distortion, while material below the band shows excessive surface segregation. The shaded band may be described as the area bounded by the following equations: Lower bound heat flux (MW/m 2 ) = 2.25 + 0.0183 ΔT f Upper bound heat flux (MW/m 2 ) = 2.86 + 0.0222 ΔT f Lower bound of alloy freezing range = 30°C Upper bound of alloy freezing range = 90°C where ΔT f is the freezing range of the alloy expressed in degree Centigrade. It is therefore preferable to employ controllable means in the belt caster for extracting heat from the metal being cast so that the rate of heat extraction for a particular alloy falls within the acceptable range. Such cooling is controlled by the belt material and texture and the thickness of a parting layer applied. Following the casting process, the thin metal strip thereby produced is normally hot and cold rolled using conventional rolling equipment to achieve the final desired gauge required by the application. At this stage, at least some of the alloys falling within the definition of Fig. 1 may be subjected to a conventional solution heat treatment and cooling to yield an Al-alloy sheet in appropriate T4 temper properties and with suitable eventual T8X temper properties. This would involve solution heat treating the cold rolled material at about 560°C in a continuous annealing and solution heat treat (CASH) line, rapidly quenching the alloy to near ambient temperature, either in forced air or water, and then naturally aging the alloy for two days or more. However, in order to obtain a desirable T4 temper properties and eventually T8X type temper properties after forming, painting and baking, it is highly desirable that at least some of the alloys having the compositions falling within the definition of Fig. 1 should be subjected to a special procedure involving solution heat treatment followed by an improved continuous controlled cooling process, as explained below. The solution heat treatment, by means of which precipitated alloying ingredients are re-dissolved in the alloy, generally involves heating the alloy sheet material to a temperature of between about 500°C and about 570°C (preferably about 560°C). The improved quenching or cooling process is then carried out. This involves cooling the alloy from the solution heat treatment temperature to an intermediate temperature without interruption and, without further interruption, cooling the aluminum alloy further to ambient temperature at a significantly slower rate. The intermediate target temperature may be approached in a single step or multiple steps. A preferred quenching process involves four uninterrupted cooling phases or sequences: first, from the solution heat treatment temperature to a temperature between about 350°C and about 220°C at a rate faster than 10°C/sec, but no more than 2000°C/sec.; second, the alloy sheet is cooled from about 350°C to about 220°C to between about 270°C and about 140°C at a rate greater than about 1°C but less than about 50°C/second; third, further cooling to between about 120°C and about 50°C at a rate greater than 5°C/min. but less than 20°C/sec; and fourth, from between about 120°C and about 50°C to ambient temperature at a rate less than about 10°C/hr. The above quenching process may be carried out with an additional step of coiling the sheet before the final step of cooling the sheet to ambient temperature at a rate less than 10°C/hour. Alternatively, the quenching process may involve forced cooling the sheet by means of water cooling, water mist cooling or forced air cooling, and coiling the sheet at a temperature of 50 to 100°C, then allowing the coil to cool at a rate of less than about 10°C/hour. The sheet most preferably exits the forced cooling at a temperature of between 120 to 150°C and the sheet is preferably coiled at a temperature of between 60°C and 85°C. When forced cooling to between 120 and 150°C is employed, the sheet is preferably passed through an accumulator in which it cools further to between 50 and 100°C and preferably 60 to 85°C, prior to coiling at that temperature. The alloys for which one of the above special quenching procedures are highly desirable, in order to develop acceptable final properties, are those previously described in connection with Fig. 1, but having Mg and Si concentrations lying within area IJKLM of the chart of Fig. 4. The area IJKLM can be approximately defined as the area contained within the following equations: Si = 0.5% (Line IJ) Mg = 0.8% (Line JK) Mg = 1.4% - %Si (Line KL) Si = 0.8% (Line LM) Mg = 0.4% (Line IM) and has Cu ≤ 2.5% In fact, for dilute alloys within the area IJKLM where Cu + Mg + Si ≤ 1.4 wt.%, the controlled quenching procedure may be essential to meet target properties for use in automotive panels. For alloys having compositions outside the volume IJKLM of Fig. 4, but otherwise within the area ABCDEF of Fig. 1, one of the special procedures is optional but desirable because improved characteristics are thereby obtained. Alloys of the preceding type lack sufficient constituent elements to develop the desired differential between T4 and T8X by conventional quenching processes that permits the formability of T4 along with the ultimate strength after paint bake. This is particularly important where the higher T8X (at least 200 MPa) is desired, or where twin belt cast material is used. Although not wishing to be bound by any theory, it is believed that when a conventional quench is used (rapid cooling to room temperature i.e. less than 45 to 50°C followed by coiling), unstable precipitates or clusters form which redissolve during the paint bake process and encourage precipitation of coarse, less defined precipitate structure. This results in a material of reduced strength. Using the slow cooling from a temperature of at least 50°C and preferably at least 60°C which is characteristic of the present invention, stable clusters form, which during paint bake promote a fine, well dispersed precipitate structure. The result of such a structure is a higher paint bake strength (T8X value). This process applies to all alloys having compositions according to this invention and therefore provides advantages, but it is particularly useful for the alloys of the range in Figure 4, and essential for the very dilute alloys. The controlled quench process wherein the sheet is coiled prior to the final cooling stage, at a temperature of between 50°C and 100°C and preferably between 60°C and 85°C brings benefits which were heretofore unrealised in the process. It is believed that the forming of a coil of metal prior to the final slow cooling stage assists in equilibrating the temperature in the coil from side to side as well as from end to end, and thus ensures that the most uniform and most desirable properties are achieved during the final slow cooling. Because of the high thermal conductance within the coil, and the relatively low surface area of the coil, this equilibration can occur. The coils may be allowed to cool naturally or fans may be used, but the equilibration still occurs because of this property, and the overall average cooling rate is still less than 10°C/hour. In order to coil the metal at a relatively higher than normal temperature, the metal must preferably leave the rapid cooling portion of the quench at a temperature of between 120 to 150°C. Additional cooling will occur during the accumulator stage prior to coiling so that the coiling temperature will fall within the desired range. The amount of cooling within the accumulator will depend on the thickness of the sheet, among other factors, but the above range generally will result in a coiling temperature which falls in the desired range. The above temperature means, however, that the accumulator itself must be specially adapted, by use, for example, of higher temperature polymer coatings on the entry rollers to the accumulator. The upper temperature for coiling may be as high as 100°C, but for some alloys within the range of this invention, such a temperature can lead to excessive development of T4 strength. The lower limit of 50°C is set so that adequate development of properties (as noted above) can occur whilst cooling to ambient. However, for some alloy combinations this temperature does not permit the full benefit to be realised, and it is therefore preferred to coil at a temperature of between 60 and 85°C to cover all alloys and conditions of the present invention. Alloy sheets prepared by the process of the invention exhibit good storage qualities, that is to say, no significant age hardening of the alloys occur during storage at ambient temperature, and they develop high yield strength by age hardening during the paint bake cycle (or a heat treatment cycle emulating the paint bake cycle for unpainted metal parts). An overall preferred process according to the present invention is shown in simplified schematic form in Fig. 5. Continuous metal strip 10, having a composition as defined in Fig. 1, is cast in twin belt caster 11 with a rate of heat extraction falling within the shaded band of Fig. 3 and subjected to hot rolling at rolling station 12. During this rolling step, some precipitates form. The hot rolled product is coiled to form coil 14. The hot rolled strip 10 is then unwound from coil 14, subjected to cold rolling in cold roll mill 15 and coiled to form coil 16. The cold rolled strip 10 is then unwound from coil 16 and subjected to a continuous solution heat treatment and controlled quenching, according to one of the three preferred cooling schemes referred to above, at station 17 to resolutionize and precipitate and constituent particles, and is then coiled to form coil 18. After natural aging for at least 48 hours, the coiled strip 18 is in T4 temper and, following normal levelling or flattening operations (not shown), may be sold to an automobile manufacturer who forms panels 20 from the strip by deformation and then paints and bakes the panels 23 to form painted panels 22 in T8X temper. The present invention is further illustrated, without limitation, by the following Examples. Example 1 (comparative) A total of 9 alloys were prepared using a pilot scale belt caster. The casting composition of these alloys is indicated in Table 2, below: Alloy # Composition (Wt%) Cu Mg Si Mn Fe 1 0.75 0.78 0.68 0.16 0.27 2 0.30 0.50 0.70 0.05 0.22 3 <0.01 0.81 0.89 0.03 0.27 4 <0.01 0.46 0.71 0.03 0.25 5 <0.01 0.61 1.20 0.001 0.18 6 0.37 0.61 1.19 -- 0.18 7 0.61 0.79 1.38 -- 0.18 8 1.03 0.99 0.29 -- 0.20 9 0.38 1.31 0.38 0.16 0.18 Alloys #1 and #3 had compositions similar to alloys for automotive sheet which have been conventionally DC cast, scalped homogenized and which, after rolling, have been subjected to conventional heat treatment and quenching. Alloy #1 was similar to AA6111, except for a higher Fe level. Alloy #3 was of similar composition to an alloy which has been produced by DC casting and formed into sheet subsequently used in automotive applications, but has no registered composition. Alloys #1, #2, #4, #8 and #9 had compositions lying in the range INAFEM of Figure 2. Alloys #2 and #4 further had compositions lying in the range IJKL of Figure 4, and Alloys #2 and #4 had Mg+Si+Cu of 1.5% and 1.2% respectively. Alloys #3 and #5 had compositions within the broad range of this invention, but outside the range INAFEM of Figure 2. Alloy #7 was selected to have a composition outside the broad range of composition of this invention. All the alloys were successfully cast on a pilot scale belt caster. The as-cast slabs were cast at a 25.4 mm gauge, 380 mm wide, at about 4m/min on copper belts. The cast slabs were reheated to 500°C and then hot rolled to 5 mm, and then cold rolled to 2.0 and 1.2 mm on a laboratory mill. The sheet was then given a simulated continuous annealing heat treatment consisting of rapid heating the material in the range 560 to 570°C, followed by a forced air quench, which simulated the conventional heat treatment given alloys of this type. After four days of natural aging (to meet the property stability requirement of T4 temper) the tensile properties were determined and some samples were given a simulated paint bake involving a 2% stretch followed by 30 minutes at 177°C (T8X temper) prior to tensile testing. The average mechanical properties of the samples are summarized in Table 3 along with properties of DC cast material for Alloys #1 (AA6111) and #3. These samples were taken after the aging normally required for stabilization of properties for this type of alloys, but prior to the flattening or levelling operation that is part of the commercial production process. Such operations can cause an increase of from 5 to 10 MPa in the T4 properties. Alloy #1 gave very comparable results to AA6111 material that had been DC cast scalped and homogenized before rolling. Alloy #3 in T4 had slightly lower yield strength and slightly higher elongation than its DC counterpart, while in T8X the properties were comparable. Belt cast alloys #1, #3, #5, #6, #8 and #9 all had T4 and T8X yield strengths within the desired ranges of 90 to 175 MPa and > 170 MPa respectively and would also fall within these ranges if allowance is made for the increase in tensile strength following normal levelling or flattening operations. Alloys #2 and #4, lying in the range IJKL of Figure 4 had yield strengths under T8X which were less than the desired 170 MPa. Alloy #7 had a yield strength under T4 which was too high to permit easy formability. Samples of all alloys except alloys #1, #3 and #4 were also subject to a simulated heat treatment corresponding to the heat treatment of this invention and consisting of a solution heat treatment as before for 5 minutes, followed by a forced air quench and immediately followed by a five hour preage at 85°C. A sample of alloy #4 was similarly processed except that an eight hour preage at 85°C was used. Tensile properties under T4 and T8X tempers were measured and are compared to the properties achieved using the conventional heat treatment in Table 4. All alloys listed, with the exception of Alloy #7, have T4 and T8X properties lying within the desired range. Alloy #7 still has T4 yield strengths which are too high for the end use, particularly if the increase for flattening or levelling noted above is added to the measured values. Alloy #4 appears to have low values of T4, but when the effects of tensile levelling are included, the T4 values lie within the acceptable range for T4. However, the T8X properties of the conventionally processed sheet lie well below the acceptable value of 170 MPa, whereas the controlled quench values exceed both the acceptable value of 170 MPa and the preferred value of 200 MPa. Example 2 (comparative). Two alloys were cast on an industrial belt caster. The slab was cast at 19 mm gauge and hot rolled to 5 mm gauge. The material was then processed in the laboratory in the same manner as indicated in Example 1. The composition of the alloys is listed in Table 5. Alloy # Composition (Wt%) Cu Mg Si Mn Fe 10 0.01 0.65 0.84 0.05 0.23 11 0.29 0.52 0.68 0.07 0.21 After four days natural age the sheet was tensile tested to obtain the T4 properties, as well given a paint bake simulation - a 2% stretch followed by 30 minutes at 177°C to obtain T8X properties. The mechanical properties in T4 and T8X tempers are listed in Table 6 and produced using the normal cooling process following solution heat treatment, which includes the data of alloys 2 and 4 of Example 1 for comparison. It should be noted that the Alloy #10 is a modified version of Alloy #4 of Example 1. Alloy #11 is equivalent to the Alloy #2 of Example 1. It can be seen that yield strength of the commercially cast Alloy #10 is higher than Alloy #4, which is expected because of the higher amounts of Mg and Si levels. The Alloy #11 has properties very similar to that of the Alloy #2 mentioned in Example 1. In all cases, the paint bake response in T8X temper is quite comparable. The alloys were also processed using the simulated controlled quench process as in Example 1. Table 7 compares tensile properties arising following the simulated conventional and simulated controlled quench process on this invention and demonstrates that the T8X properties can be increased to target levels by the process on this invention. The T4 yield strengths are also reduced, but as noted in Example 1, when consideration is made of the normally higher values obtained following commercial processes of tensile levelling for example they still fall within the desired range of properties, and both T4 and T8X properties are consistent with the results of Example 1. Example 3 (comparative) Alloys #10 and #11 of Example 2 were also processed, following belt casting and hot rolling, on a commercial cold mill and continuous heat treatment line. The heat treatment line used the solution heat treatment and controlled quench process of this invention, specifically using four temperature steps during cooling with a coiling step prior to the final cooling step. The coils underwent the normal ageing of at least 48 hours. Samples were taken for testing, however, prior to any flattening or levelling operation. The tensile properties of the samples are given in Table 8. The tensile properties differ slightly from the properties for simulated controlled quench material from Example 2, because the simulation does not exactly duplicate the commercial process. However the tensile properties under T4 and T8X fall within the requirements of invention. Alloy # Dir. T4 T8X YS (MPa) UTS (MPa) %El YS (MPa) UTS (MPa) %El 10 L 112.0 213.4 19.9 - - - T 107.5 210.2 21.8 234.8 288.0 14.2 11 L 103.5 209.2 21.9 - - - T 99.9 210.7 27.5 221.7 281.4 16.4 Example 4 (invention) Five alloys within the composition range of this invention were DC cast in commercial size ingots. The casting composition of these alloys is indicated in Table 9. The ingots were scalped, homogenized for several hours at 560°C, hot and cold rolled to a finished gauge. The sheet was solution heat treated and quenched according to the process of this invention, with the quench process involving forced cooling followed by coiling at different temperatures as given in Table 10. Table 10 also summarizes the tensile properties of the resulting materials. The T4 properties are measured under the same conditions as outlined in Example 1. All the alloys after controlled quench had T4 and T8X properties within the range indicated in Table 1. However, Alloy 13, when coiled at a temperature of 90°C (achieved by using thicker strip which therefore had a smaller temperature drop in the accumulator stage), had a T4 value approaching the upper limit of acceptability, particularly if corrected for stretching (as described in Example 1). For other alloys the effect of higher coiling temperature on T4 is not expected to be as severe, but nevertheless an upper limit for coiling temperature of 85°C is more preferred. For alloys 12 to 15, laboratory cast samples of the same composition were prepared and processed to sheet. The sheet was given a simulated heat treatment and conventional quench as in Example 1. The T8X properties of these comparative samples were clearly less than those which had been quenched using the process of this invention, and although they fell within the broadest acceptable range of T8X, they did not meet the more stringent requirements of T8X of at least 200 MPa. Alloy 16 was processed in two ways after cooling. In one case the coil was insulated and in the other case the coil was cooled using fans. The T4 and T8X properties were essentially the same and fell within the desired ranges. Alloy 12 which has a very similar composition was cooled, after coiling, by standing in ambient air and the values are once again comparable. Final stage cooling in coil form, as long as the overall rate of cooling is less than 10°C/h is independent of the way the exterior of the coil is handled, indicating that the internal equilibration is sufficiently fast to ensure thermal uniformity and desirable properties. Alloy Cu Mg Si Fe Mn others Al 12 0.76 0.79 0.64 0.23 0.19 <0.1 Bal 13 0.40 0.39 1.27 0.19 0.07 <0.1 Bal 14 0.80 0.42 0.99 0.21 0.05 <0.1 Bal 15 0.50 1.0 0.49 0.25 0.07 <0.1 Bal 16 0.72 0.71 0.63 0.13 0.14 <0.1 Bal

Claims: A process of imparting T4 and T8X temper yield strengths suitable for automotive applications to a sheet of an aluminum alloy, said T4 temper yield strength, after natural aging and levelling or flattening, being in the range of 90 to 175 MPa, and said T8X temper yield strength being at least 170 MPa; and said aluminum alloy containing magnesium, silicon and copper in amounts in percent by weight falling within the area ABCDEF of Figure 1 of the accompanying drawings, and amounts of copper between the contours shown by broken lines in Figure 1 and 0.3 wt.% in area BHGI and 0 wt.% in areas HAFG and IEDC of Figure 1; and optionally at least one additional element selected from Fe in an amount of 0.4 percent by weight or less, Mn in an amount of 0.4 percent by weight or less, Zn in an amount of 0.3 percent by weight or less; and optionally at least one other element selected from Cr, Ti, Zr and V, the total amount of Cr + Ti + Zr + V not exceeding 0.3 percent by weight of the alloy: the balance being Al; in which process the sheet is subjected to a procedure selected from (a) solution heat treating the sheet at a temperature in the range of 500 to 570°C and then cooling the sheet according to a scheme comprising cooling to between 350°C and 220°C at a rate greater than about 10°C/sec but not more than about 2000°C/sec, then cooling to a temperature in the range of 270°C and 140°C at a rate greater than 1°C/sec but not faster than 50°C/sec, then cooling to between 120°C and 50°C at a rate greater than 5°C/min, but less than 20°C/sec, and then cooling to ambient temperature at a rate of less than about 10°C/hour; or (b) solution heat treating the sheet at a temperature in the range of 500 to 570°C and then forced cooling the sheet using a means of cooling selected from water, water mist or forced air, to a temperature of between 50 and 100°C, then allowing the sheet to cool at a rate of less than about 10°C/hour; characterized in that , when the sheet is at a temperature of between 50 to 100°C, the sheet is coiled and the coil is cooled or allowed to cool at said rate of less than about 10°C/hour. A process according to claim 1, characterized in that the sheet is cooled to a temperature in the range of 120 to 150°C, then passed through an accumulator where the sheet is additionally cooled to a temperature of 50 to 100°C prior to being coiled at said temperature of between 50 and 100°C. A process according to claim 1 or claim 2, characterized in that the sheet is coiled at a temperature of between 60 and 85°C. A process according to claim 1, claim 2 or claim 3, characterized in that said procedure is carried out on said sheet of said aluminum alloy in which the total amount of the at least one other element, Cr + Ti + Zr + V, does not exceed 0.15 percent by weight of the alloy. A process according to claim 1, claim 2, claim 3 or claim 4, characterized in that said procedure is carried out on said sheet of said aluminum alloy which contains amounts of Mg and Si falling within area INAFEM of Figure 2 of the accompanying drawings. A process according to claim 1, claim 2, claim 3 or claim 4, characterized in that said procedure is carried out on said sheet of said aluminum alloy which contains amounts of Mg and Si falling within area IJKLM of Figure 4 of the accompanying drawings.

Inventors: BULL MICHAEL JACKSON, FITZSIMON JOHN, GUPTA ALOK KUMAR, JIN ILJOON, LLOYD DAVID JAMES, MAROIS PIERRE H, BULL, MICHAEL JACKSON, FITZSIMON, JOHN, GUPTA, ALOK KUMAR, JIN, ILJOON, LLOYD, DAVID JAMES, MAROIS, PIERRE H.
Applicants: ALCAN INT LTD, ALCAN INTERNATIONAL LIMITED
Application year: 1995
Main classification: C22C21
IPC codes: C22C21, C22F1