Abstract:
A metal sheet for a motor vehicle body outer panel, having a thickness ranging between 0.8 and 1.2 mm, containing, in wt. %, Fe 0.25-0.40 and preferably 0.25-0.35; Si 0.90-1.20 and preferably 0.95-1.10; Cu 0.10-0.25 and preferably 0.15-0.20; Mg 0.35-0.50 and preferably 0.40-0.50; Mn 0.05-0.20 and preferably 0.08-0.15; other elements&lt;0.05 each and &lt;0.15 in total, the rest being aluminum. The sheet has, after solution heat treatment, quenching, pre-tempering or reversion, and maturation at room temperature for 3 weeks to 6 months, an L R 0.2  direction yield strength less than 160 MPa, and preferably less than 150 MPa. A yield strength&gt;180 MPa can be obtained on the body stamping part after the paint has been cured. The sheet of the invention enables a reduction in the thickness of parts while satisfying all the other required properties.

Description:
DOMAIN OF THE INVENTION  
       [0001]     The invention relates to the domain of Al-Si-Mg alloy sheets, more particularly made of a 6016 type alloy according to the Aluminum Association, to be used for manufacturing by drawing of car body outer panel parts such as wings, doors, tailgates, bonnets and roofs.  
       STATE OF THE ART  
       [0002]     Aluminium is increasingly used in automobile construction to reduce the weight of vehicles and thus reduce fuel consumption and the release of pollutants and greenhouse gases. Sheets are used particularly for making body outer panel parts, particularly doors. This type of application requires a number of properties that are sometimes contradictory, for example: 
        good formability for drawing and hemming operations,     controlled yield strength in the delivery state of the sheet, to control springback,     high mechanical strength after baking the paints to obtain good dent resistance while minimizing the weight of the part,     good resistance to corrosion of the painted part, particularly filiform corrosion,     good surface quality after shaping and painting,     good behaviour in miscellaneous assembly processes used in automobile bodywork such as spot welding, laser welding, gluing, clinching and riveting,     compatibility with requirements for recycling of manufacturing waste or recycled vehicles,     an acceptable cost for large series production.        
 
         [0011]     Al-Mg-Si alloys in the 6000 series have been chosen to satisfy these requirements. In Europe, the 6016 and 6016A alloys with thicknesses of the order of 1 to 1.2 mm are most frequently used for this application since they give a better compromise between the various required properties, particularly enabling better formability, particularly for hemming, and better resistance to filiform corrosion than alloys with a higher copper content such as the 6111 alloy that is widely used in the United States. 6016 type alloys are described particularly in Alusuisse&#39;s patent FR 2360684 and the applicant&#39;s patent EP 0259232, while 6111 type alloys are described in Alcan International Limited&#39;s patent U.S. Pat. No. 4,614,552. Alloys with low iron content (&lt;0.2%) like those described in Alcoa&#39;s patents U.S. Pat. No. 5,525,169 and U.S. Pat. No. 5,919,323 are also known, and an alloy of this type was registered as 6022. The compositions (% by weight of the main elements) of the 6016, 6016A, 6022 and 6111 alloys registered at the Aluminum Association are indicated in table 1:  
                                   TABLE 1                       Alloy   Si   Fe   Mg   Cu   Mn                   6016   1.0-1.5   &lt;0.5   0.25-0.6   &lt;0.2    &lt;0.2       6016A   0.9-1.5   &lt;0.5    0.2-0.6   &lt;0.25   &lt;0.2       6022   0.8-1.5   0.05-0.2   0.45-0.7   0.01-0.11   0.02-0.10       6111   0.5-1.0   &lt;0.4    0.5-1.0   0.5-0.9   &lt;0.4                  
 
         [0012]     However, the mechanical strength of the 6016 alloy after the paint has been baked, and therefore the dent resistance, remains significantly lower than the corresponding values for the 6111 alloy, particularly since the baking temperature is tending to go down such that hardening during ageing is less effective. This is why automobile manufacturers are asking for a higher mechanical strength after painting.  
         [0013]     In order to achieve this, the applicant has developed new variants of the 6016 alloy, and particularly a “DR120” variant leading to a yield strength in the T4 temper of the order of 120 MPa. These developments were covered in publications, particularly the articles by R. Shahani et al. “Optimised 6xxx aluminium alloy sheet for autobody outer panels” Automotive Alloys 1999, Proceedings of the TMS Annual Meeting Symposium, 2000, pp. 193-203, and D. Daniel et al. “Development of 6xxx Alloy Aluminium Sheet for Autobody Outer Panels: Bake Hardening, Formability and Trimming Performance” IBEC&#39;99—International Body Engineering Conference, Detroit, 1999, SAE Technical Paper No. 1999-01-3195.  
         [0014]     Alcan has proposed a new variant of the 6111 alloy called 6111-T4P, which has an improved yield strength after baking the paint (typically 270 to 280 MPa) without reducing formability in the T4 temper. In particular, this product has been described in the article by A. K. Gupta et al. “The Properties and Characteristics of Two New Aluminium Automotive Outer Panel Materials”, SAE Technical Paper 960164, 1996. The article also mentions a new alloy temporarily called 61XX-T4P, for which the composition has not been divulged, which has a lower yield strength in the T4 state than the conventional 6111-T4, with a similar response to paint baking.  
         [0015]     These new developments all include optimised pre-ageing type heat treatment, carried out after quenching to improve hardening while baking paint. If this type of treatment is not performed, the hardening rate while baking reduces as the waiting time at ambient temperature between quenching and baking increases, and a waiting time of several weeks is almost inevitable in industrial production. This phenomenon has been known for a long time and has been described for example in the article by M. Renouard and R. Meillat “Le prérevenu des alliages aluminium-magnésium-silicium (Preliminary annealing of aluminium-magnesium-silicon alloys)”  Mémoires Scientifiques de la Revue de Métallurgie  (Scientific Memories of the Metallurgy Review), December 1960, pp. 930-942.  
         [0016]     To avoid the unfavourable effect of waiting, it is necessary to either perform pre-ageing by staged quenching or a heat treatment immediately after quenching, or to store the metal in a freezer which is not very convenient for automobile body parts, or to perform a reversion treatment.  
         [0017]     The pre-ageing temperature and duration for 6000 alloys are described for example in the article by R. Develay “Traitements thermiques des alliages d&#39;aluminium (Heat treatments for aluminium alloys)”,  Techniques de l&#39;Ingénieur  (Engineering techniques), section M 1290, 1986, in the article by D. W. Pashley et al. “Delayed ageing in aluminium-magnesium silicon alloys: effect on structure and mechanical properties”, Journal of the Institute of Metals, No. 94, 1966, pp. 41-49, and in patent EP 0480402 (Surnitomo Light Metal). Patent FR 1243877 (Cegedur) also describes a continuous furnace that can be used for pre-ageing.  
         [0018]     Considering the increased use of aluminium alloy sheets for mass-produced automobile body outer panels, there is still a demand for even more improved grades to reduce thicknesses without adversely affecting other properties. The reduction in thickness is usually limited by the lack of stiffness of the formed part, this limit being equal to the thickness of the equivalent part made of steel multiplied by 1.4. Therefore, sheets must be capable of achieving a dent resistance on a formed part after the paint has been baked equal to at least the resistance for steel parts, for an aluminium/steel thickness ratio of 1.4, while maintaining good drawability and hemming ability.  
       SUBJECT OF THE INVENTION  
       [0019]     The purpose of this invention is to provide 6016 type alloy sheets for automobile body outer panels with a composition adapted to recycling, sufficient formability for deep drawing and hemming under severe conditions, improved resistance to indentation compared with sheets of the 6016 type according to the prior art while controlling springback, with good gluing properties, cutting without the formation of slivers, and good resistance to filiform corrosion. The subject of the invention is a sheet for an automobile body outer panel part between 0.8 and 1.2 mm thick, and the following composition (% by weight):  
                                                   Fe: 0.25-0.40 and preferably: 0.25-0.35           Si: 0.90-1.20 and preferably: 0.95-1.10           Cu: 0.10-0.25 and preferably: 0.15-0.20           Mg: 0.35-0.50 and preferably: 0.40-0.50           Mn: 0.05-0.20 and preferably: 0.08-0.15                      
        other elements&lt;0.05 each and &lt;0.15 total, the remainder being aluminium,     presenting a yield strength R 0.2  in the L direction less than 160 MPa and preferably less than 150 MPa after solution heat treatment, quenching, pre-ageing or reversion and ageing at ambient temperature for between 3 weeks and 6 months. The yield strength of the drawn part after a heat treatment corresponding to baking the paint is greater than 180 MPa and preferably greater than 200 MPa.        
 
     
    
     DESCRIPTION OF THE INVENTION  
       [0022]     The invention is based on a narrow composition range within the definition of the 6016A composition registered at the Aluminium Association, in order to obtain all the required properties.  
         [0023]     The silicon content is near the bottom part of the content range of 6016A, while the magnesium content is at the centre of the range. This drop in the silicon content contributes to more complete solution heat treatment of the alloy, favourable for formability. The iron content remains above 0.25% which, unlike low iron grades such as 6022, enables the use of recycled metal and results in a better surface appearance after drawing.  
         [0024]     The copper content is controlled within very narrow limits: a content of at least 0.1%, slightly more than the content for existing 6016 or 6022 grades, contributes to the mechanical strength, but above 0.25% there is a risk of filiform corrosion of the alloy. The alloy must contain at least 0.05% of manganese, chromium, vanadium or zirconium to control the grain size and prevent the appearance of orange peel during severe deformations, for example such as hemming used for bonnets. Conversely, if the total content of these elements exceeds 0.20%, it is bad for formability.  
         [0025]     The sheet manufacturing process according to the invention typically includes casting of a plate, possibly scalping of this plate and homogenisation by simply heating it to a temperature of between 400 and 570° C. for between 6 and 24 h. Hot rolling preferably takes place at an input temperature of more than 510° C., which contributes to making the mechanical strength better than what would be obtained at a lower input temperature. The winding temperature of the hot rolled strip must be less than 350° C., and preferably 300° C., to guarantee mechanical characteristics and to avoid any ridging defects. The hot rolled strip is then cold rolled down to the final thickness, possibly with intermediate annealing at a temperature of between 300 and 450° C. if it is done in a batch furnace, or between 350 and 570° C. if is done continuously. The last cold rolling pass may be made with a textured cylinder, for example by electron beam treatment (EBT), electro-erosion (EDT), or by laser beam which improves the formability and surface appearance of the part formed after painting.  
         [0026]     It is also possible to use strips obtained directly by continuous casting, either by twin-roll casting or twin-belt casting, and to perform cold rolling and subsequent operations under the same conditions.  
         [0027]     The solution heat treatment takes place at a temperature above the alloy solvus temperature while avoiding overheating. The composition according to the invention is capable of very complete solution heat treatment, resulting in an almost complete absence of silicon type phases in the microstructure and by a very small peak area, less than 1 J/g in the 565-580° C. range in a differential enthalpy analysis diagram, the test being carried out at a temperature rise rate of 20° C./min.  
         [0028]     After the solution heat treatment, the sheet is quenched, usually with cold water or air. Quenching may be followed immediately by a pre-ageing type heat treatment like that described in the prior art mentioned above, in order to improve hardening performances of the paint during baking.  
         [0029]     Pre-ageing is not necessary isothermal and its duration depends on the temperature. This can be taken into account by defining an equivalent time t eq  using the formula:  
         T   ⁡     (   eq   )       =       ∫       exp   ⁡     (       -   6000     /   T     )       ⁢     ⅆ   t           exp   ⁡     (       -   6000     /     T   ref       )             
        where T (in ° K) is the temperature and t is the pre-ageing time, T ref  is a reference temperature of 373° K, namely 100° C. It is known that if pre-ageing is to be efficient, it must be done at a temperature of more than 50° C. for a time of between 0.3 and 20 h. If the equivalent time is insufficient, the hardening rate of paint while baking reduces with the waiting time at ambient temperature. On the other hand, if the equivalent time is too long, the mechanical characteristics increase too much during pre-ageing and the formability of the sheet is degraded. For 6016 type alloys, an equivalent time of 1 to 10 h, and preferably 3 to 6 h, is quite suitable.        
 
         [0031]     The sheet is usually stored for a variable time period at this stage, which leads to natural ageing that increases the yield strength with time. After three weeks of ageing, the thickness of sheets according to the invention is of the order of 0.9 to 1 mm, the yield strength in the L direction is of the order of 130 MPa which is higher than all 6016 variants, including high strength grades DR100 and DR120 described in the article by R. Shahani et al. mentioned above, and only slightly lower than the value for 6022. After six months ageing, this yield strength remains below 160 MPa or even 150 MPa, unlike 6022 and 6111 alloys. This special feature enables control over springback during forming which becomes increasing difficult to control when thicknesses are reduced and when the yield strength is increased, so that many iterations are necessary when developing stamping tools. Before forming, the sheet may be coated with a lubricant (oil or dry lubricant) adapted to drawing, assembly and surface treatment of the part to be made.  
         [0032]     Sheets according to the invention have formability as measured by the LDH 0  (“Limiting Dome Height” in plane deformation) parameter that is better than the 6111 and 6022 alloys and as good as high strength 6016 grades.  
         [0033]     The LDH parameter is broadly used for evaluating the drawability of 0.5 to 2 mm thick sheets. Many publications have been made, particularly the publication by R. Thomson, “The LDH test to evaluate sheet metal formability—Final Report of the LDH Committee of the North American Deep Drawing Research Group”, SAE Conference, Detroit, 1993, SAE Paper No. 930815.  
         [0034]     The LDH test is a drawing test on a blank blocked by a retaining ring around its periphery. The pressure of the blank clamp is controlled to prevent sliding in the retaining ring. The 120×160 mm blank is strengthed in a mode similar to plane deformation. Lubrication between the stamp and the sheet is achieved using a plastic film and grease (Shell HDM2 grease). The punch lowering speed is 50 mm/min. The LDH value is the displacement of the punch at failure, namely the drawing limiting depth. The average of three tests is determined, giving a confidence range of 95% on a measurement of ±0.2 mm.  
         [0035]     Sheets according to the invention have better crimpability than 6111 or 6022 alloy sheets, and the crimpability is just as good as high strength 6016 alloy sheets according to the prior art. This hemming ability is evaluated by a laboratory test including flanging at 90°, pre-hemming at 45° and final flat hemming.  
         [0036]     Sheets according to the invention also have a very small deformation anisotropy that can be measured by the difference between the LDH for a principal deformation parallel to the rolling direction, and a principal deformation perpendicular to the rolling direction. This difference is less than 1 mm and preferably less than 0.6 mm.  
         [0037]     The body outer panel part is usually made by cutting out a blank in the sheet, drawing this blank and trimming it with the press. During stamping, it is essential to avoid the occurrence of roping or ridging, which deteriorates the final paint appearance and can reduce formability, particularly in the case of severe deformation in the direction perpendicular to the rolling direction. Different means have been provided for this purpose, for example controlling the hot rolling outlet temperature between 270 and 340° C., as indicated in the applicant&#39;s patent EP 0259232. It is also important to avoid the occurrence of “orange peel” stamping which contributes to a visible appearance defect after painting. This is done preferably by keeping the grain size smaller than 50 μm, which can be achieved by the presence of a sufficient quantity of manganese in the alloy, or other elements playing a similar role such as chromium, vanadium or zirconium, by temperature control and by the time of the solution heat treatment and by a sufficient reduction, typically at least 30% by cold rolling. For some parts such as bonnets, the edges of the drawn blank are flanged at 90° and a lining stamping is inserted on which pre-hemming is done followed by final flat hemming.  
         [0038]     It is also necessary to avoid the formation of slivers during blank cutting and turning operations after drawing, these slivers possibly causing the appearance of defects requiring manual touching up. The design of the cutting tool is important in this respect, and recommendations were made in the article by D. Daniel et al. mentioned above.  
         [0039]     After stamping and possibly hemming, the part is covered by one or more coats of paint, with a baking step following each coat. The critical step is baking of the cataphoresis layer, which is usually done at a temperature of between 150 and 200° C. for 15 to 30 minutes. The baking temperature rarely exceeds 170° C. if there is no cataphoresis. Paint baking contributes towards an ageing treatment of the part. The yield strength of the part made with a sheet according to the invention, with baking for 20 minutes at 165° C., is higher than 180 MPa and often higher than 200 MPa. Thus, with a part made from a 0.9 mm thick sheet, the resistance to dynamic indentation is similar to the value for a part made from a typical body steel sheet with a yield strength of the order of 250 to 300 MPa and 0.7 mm thick, which is not the case for other 6016 grades.  
         [0040]     Sheets according to the invention can be used to perform different operations routinely performed for making car body outer panel parts, such as hemming, clinching, riveting, spot welding, laser welding and gluing. In particular, it is possible to glue hemmed joints, used particularly in making bonnets, without firstly performing a chemical surface treatment such as chemical conversion or passivation, for example using phospho-chromic compounds, or products based on titanium, zirconium or silanes.  
         [0041]     Parts made from sheets according to the invention also have good resistance to filiform corrosion after painting, better than alloys with a high copper content like 6111 alloy.  
         [0042]     For economic reasons, it may be useful to combine steel structures and aluminium body outer panel parts on the same vehicle, for example for wings, roofs and doors. In this type of assembly, the major difficulty is with the management of differences in thermal expansion between two materials when the paint is being baked, particularly during the cataphoresis baking which is usually done at between 160 and 200° C. It is essential to limit residual deformations after baking to a level acceptable for the appearance of the vehicle.  
         [0043]     Sheets according to the invention can limit these deformations, independently of the geometry of the parts and the assembly mode chosen. The applicant thus demonstrated that a high yield strength at the baking temperature, for example more than 140 MPa at a temperature of 160° C. for the alloy according to the invention, had a favourable effect on the deformation level, if the assembly is made after baking, and it is thus preferable to limit the baking temperature.  
         [0044]     Other factors may also limit deformations, for example the presence of ribs designed to stiffen the aluminium panel, or the spacing between assembly points. An assembly with a continuous link such as gluing could also be used, with at least a partial polymerisation of the glue before baking, or a transparent laser welding.  
       EXAMPLES  
     Example 1  
       [0045]     500 mm thick plates were cast of 8 alloys A to I with the composition (% by weight) as indicated in table 1: Table 1 
                                                                         TABLE 1                                   Alloy   Si   Fe   Cu   Mn   Mg                                        A   1.15   0.31   0.07   0.10   0.41           B   1.0   0.29   0.09   0.11   0.33           C   0.58   0.26   0.79   0.10   0.73           D   0.58   0.26   0.79   0.10   0.73           E   1.22   0.13   0.07   0.08   0.56           F   1.0   0.30   0.35   0.15   0.45           G   1.0   0.30   0.18   0.30   0.45           H   1.0   0.30   0.18   0.05   0.45           I   1.0   0.30   0.18   0.15   0.45                      
 
         [0046]     Composition A represents a conventional 6016 alloy, B is the applicant&#39;s grade DR100 described in the articles mentioned above, C and D are a 6111 alloy, E is a 6022 alloy, F, G, H and I are alloys with similar compositions, differing either by Cu (F), or by Mn (G and H) from the composition I according to the invention.  
         [0047]     The plates were scalped and homogenised for 10 h at 570° C., and then hot rolled directly on homogenisation heat, firstly on a reversible mill, then on a tandem mill. The lamination start temperature was of the order of 540° C., and the hot strip winding temperature was of the order of 310° C.  
         [0048]     The hot rolled strip rolled to 3 mm is then cold rolled to the final thickness of 1 mm. An intermediate annealing is carried to a thickness of 2.5 mm, consisting of either a “batch” annealing in a coil with a temperature rise to 350° C. in 10 h, 2 h waiting time followed by slow cooling, or a “flash” annealing in a continuous furnace with a temperature rise to 400° C. in about one minute and immediate cooling. Samples taken from the strips are subjected to a solution heat treatment at a temperature of 570° C. for less than one minute, and are then quenched in cold water. Complementary treatment for 2 h at 100° C. in the oil bath immediately after quenching to simulate industrial pre-ageing, is applied to samples made of alloys B, D, F, G, H and I.  
         [0049]     The yield strength R 0.2  (in MPa) was measured in the L direction after 3 weeks and 6 months ageing at ambient temperature, followed by an ageing treatment for 30 minutes at 165° C. or 185° C., simulating the paint baking treatment. The formability was also measured using the LDH parameter (in mm), the principal deformations being parallel to and perpendicular to the rolling direction respectively. The results are given in table 2:  
                                                                                                 TABLE 2                                   A   B   C   D   E   F   G   H   I                                    Inter.   batch   batch   batch   batch   batch   flash   flash   flash   flash       an-       nealing       R 0.2     121   101   140   139   158   129   123   125   124       3 weeks       R 0.2     133   112   152   156   173   144   138   142   145       6       months       R 0.2     135   157   160   221   163   194   187   189   191       165° C.       R 0.2     159   186   190   258   190   228   222   225   225       185° C.       LDH //   27.4   29.0   25.4   26.1   25.9   27.2   25.9   27.5   28.3       LDHL   27.4   28.4   25.6   27.0   25.7   25.3   26.3   26.9   28.2       perp.                  
 
         [0050]     It was found that after 3 weeks of ageing, sample I according to the invention has the same order of yield strength as the conventional 6016 (sample A) and significantly lower than the corresponding value of the 6111 alloys (C and D) and the 6022 alloy (E). The position of the yield strength of sample I with respect to other alloy samples has not changed after 6 months of ageing.  
         [0051]     Formability, as measured by the LDH parameter, is practically as good as the formability of the best alloy, which is the DR100. Moreover, measured values of LDH in the rolling direction and in the direction perpendicular to the rolling direction are practically identical, which is not always the case for other samples, which enables good isotropy in forming.  
         [0052]     Conversely, the yield strength of sample I after the paints have been baked following a pre-ageing is high, significantly higher than the value for the 6016 and DR100 alloys, of the same order as the value for alloy F which has a higher content of copper, and is intermediate between the values for the two 6111 grades, guaranteeing high dent resistance of the finished part.  
         [0053]     The behaviour during hemming was also measured on 1 mm thick sheets in the direction parallel to rolling and in the direction perpendicular to rolling, the resistance to filiform corrosion after phosphating, cataphoresis and painting and the occurrence or lack of occurrence of slivers of filaments when cutting out or trimming after drawing.  
         [0054]     The hemming test is done in three operations: flanging of edges at 90° C., pre-hemming at 45° and hemming flat on a 0.7 mm thick lining plate. The hemmed edges are then classified by visual inspection, as described in the article by D. Daniel et al. in IBEC 99.  
         [0055]     The resistance to filiform corrosion is evaluated according to standard EN 3665, with sample size 150×60×1 mm after painting and scratching. The test procedure includes corrosion activation by HCl vapour for 1 h, followed by exposure in a wet room at 40° C. for 1000 h. The maximum length of corrosion filaments is measured, taking an average of 3 test pieces per case with the following classification: &lt;2 mm=good; 2-5 mm=medium; &gt;5 mm=poor. The cut test is described in the article by D. Daniel et al in IBEC 99 mentioned above. The clearance was 10% of the thickness and the cutting angle was 0°.  
         [0056]     The results are given in table 3:  
                                                                                                 TABLE 3                                   A   B   C   D   E   F   G   H   I                                    hemm   good   good   crack   crack   crack   orange   good   good   good       //                       skin       hemm   good   good   crack   crack   orange   orange   start   orange   good       perp                   skin   skin   crack   skin       Fil.   good   good   bad   bad   good   medium   good   good   good       Corr.       Slivers   no   no   filaments   filaments   slivers   no   no   no   no                  
 
         [0057]     It was found that the behaviour of sample I is satisfactory for these various criteria, so that body outer panel parts with an impeccable appearance can be made.  
       Example 2  
       [0058]     Aluminium alloy panels were made with the composition indicated in table 4, with a manufacturing procedure similar to that in example 1, possibly but not necessarily comprising pre-ageing and heat treatment after forming and before assembly, as also mentioned in table 4. The panel dimensions are 1.6 m×0.9 m.  
                                                                                     TABLE 4                                                   T eq                     Si   Fe   Mg   Cu   Mn   pre-       Sample   Alloy   (%)   (%)   (%)   (%)   (%)   ageing   Anneal.                                J   Inv.   1.05   0.25   0.45   0.19   0.14   5 h   4-185°       K   Inv.   1.05   0.25   0.45   0.19   0.14   5 h   No       L   6111   0.70   0.25   0.60   0.69   0.21   —   No       M   DR100   1.03   0.26   0.32   0.07   0.11   5 h   No       N   6016   1.03   0.26   0.32   0.07   0.11   —   No                  
 
         [0059]     For each alloy, three panels were tested with different geometries each comprising ribs obtained by folding and parallel to the short side of the rectangle.  
         [0060]     These panels were riveted on rectangular steel frames to simulate the case of body outer panel sheets made of aluminium alloy on a steel structure of a vehicle. The assembly is made by riveting at a pitch of 50 mm on the long sides of the rectangles. The residual deformations of the panels were observed after 20 minutes heat treatment at 160° C. to simulate cataphoresis baking. The mechanical properties (ultimate strength R m  and yield strength R 0.2  (in MPa)) of panels at ambient temperature and at the baking temperature of 160° C. were also measured, for a temperature rise rate equal to about 20° C./min. Table 5 contains the results.  
                                                                             TABLE 5                                       R 0.2                         Panel 1   Panel 2   Panel 3   ambi-   R m     R 0.2     R m         Samp.   deform.   deform.   deform.   ent   ambient   160° C.   160° C.                                J   Very           261   321   239   250           low       K   Low   Very   Very   160   282   148   208               low   low       L   High           164   309   137   220       M   High   Low   Very   142   263   127   186                   low       N   Very           122   230   106   161           low                  
 
         [0061]     It is found that the alloy according to the invention effectively reduces residual thicknesses after baking. The performance of alloys is clearly correlated with the yield strength at the baking temperature. Finally, heat treatment before assembly and the addition of ribs are beneficial in reducing deformations.  
       Example 3  
       [0062]     An evaluation was made of the resistance to dynamic indentation of a 1 mm thick sheet produced according to a manufacturing procedure of the type shown in example 1, comprising pre-ageing for an equivalent time of 5 h, a 20-minute heat treatment at different temperatures simulating baking of the paint, made of an alloy according to the invention and a 6016 DR100 alloy, and was compared with the corresponding values of a 0.7 mm-thick steel sheet with a yield strength equal to 290 MPa after baking the paint. This value of 290 MPa for the yield strength of a steel sheet for use in an automobile body after baking corresponds approximately to the average of yield strengthes of steel sheets used for body outer panels for the most frequently used recent European cars. A 1 mm-thick aluminium sheet can resist about 50% more elongation than a 0.7-mm thick steel sheet.  
         [0063]     The device used for the indentation test comprises a 15 mm-diameter indenter weighing 138 g, released from a height of 1 m at a speed of about 16 km/h, onto the sample sheet clamped between two steel plates. The permanent indentation depth is measured (in mm). The results are given in table 6.  
                                                                     TABLE 6                           R 0.2             Indent.               Baking   Inv   R 0.2     R 0.2     Inv   Indent.   Indent.       temperature   all.   DR100   steel   all.   DR100   steel                                170° C.   193   161   290   1.55   1.80   1.45       185° C.   217   189   290   1.45   1.62   1.45       205° C.   230   207   290   1.38   1.46   1.45                  
 
         [0064]     It was found that for a paint baking temperature equal to 185° C., the 1 mm-thick sheet according to the invention has the same dent resistance as the 0.7-mm thick steel sheet. For the DR100 alloy, this is only true for a paint baking temperature of 205° C., which is higher than temperatures usually used by automobile manufacturers. A stronger alloy such as 6111 would increase the dent resistance to exceed market needs, but this would be to the detriment of formability, particularly during hemming.