Process for manufacturing a 6xxx alloy sheet having an excellent surface quality

Abstract

The present invention discloses a process for manufacturing a 6xxx alloy sheet that is in the T4 state and comprises 0.10% to 0.40% Fe, 0.05% to 0.20% Mn and 0.01% to 0.04% Cr. The process involves cooling after homogenization, two-stage hot rolling, intermediate recrystallization during cold rolling. The sheet according to the invention affords a compromise between bending and surface waviness in order to obtain an improved quality after painting.

Description

[0001] DESCRIPTION

[0002] Title: PROCESS FOR MANUFACTURING A 6XXX ALLOY SHEET WITH EXCELLENT

[0003] SURFACE QUALITY

[0004] TECHNICAL FIELD

[0005] The invention relates to the field of aluminum alloy sheets intended for the stamping manufacture of body parts for the body-in-white of motor vehicles.

[0006] PREVIOUS ART

[0007] Aluminum alloys are increasingly used in automotive construction to reduce vehicle weight and thus reduce fuel consumption and greenhouse gas emissions.

[0008] Aluminum alloys are used in particular to produce bodywork parts, especially visible bodywork parts, especially external ones. Visible bodywork parts, especially external ones, are subject to technical specifications so that these parts, after painting, have the required appearance to satisfy motorists. Conventionally, the surface quality is characterized by measuring the roping, but this characterization does not highlight all the surface defects.

[0009] Application EP1967598 discloses a sheet of a 6000 type aluminum alloy containing Si and Mg as the main alloying components and having excellent formability sufficient to enable flat bending, excellent dent resistance, and good hardenability during paint baking. The application discloses a method for producing the aluminum alloy sheet, which comprises subjecting an ingot to a homogenization treatment, cooling it to a temperature below 350°C at a cooling rate of 100°C / h or more, optionally to room temperature, reheating it to a temperature of 300 to 500°C and subjecting it to hot rolling, cold rolling the hot-rolled product, and subjecting the cold-rolled sheet to a solution treatment at a temperature of 400°C or more, followed by quenching,

[0010] Application EP3485055 discloses a method for producing a 6xxx series aluminum sheet comprising the steps of homogenizing a 6XXX series aluminum alloy ingot; cooling the homogenized ingot at a cooling rate of between 150°C / h and 2000°C / h directly to the hot rolling start temperature; hot rolling the ingot to a final hot rolling thickness and coiling to the final hot rolling thickness with conditions such that at least 50% recrystallization is obtained; cold rolling to obtain a cold rolled sheet. The method according to the invention is particularly useful for manufacturing sheets for the automotive industry which combine high tensile strength and good formability properties suitable for cold stamping operations, as well as high surface quality and high corrosion resistance with high productivity.

[0011] Application WO2018 / 206696 discloses a method for manufacturing an aluminum alloy rolled sheet having excellent formability and good paint bake hardenability, comprising: (a) casting an ingot of an Al-Si-Mg aluminum alloy comprising, in wt%, Si 1.0% to 1.50%, Mg 0.10% to 0.40%; (b) heating the ingot to a temperature above 550°C; maintaining the ingot at a temperature above 550°C for at least about 4 hours; cooling the ingot to a temperature between 460°C and 520°C; and maintaining the ingot at a temperature between 460°C and 520°C for less than 6 hours; (c) hot rolling the ingot in one or more rolling stages to an intermediate gauge of between 15 mm and 40 mm and in which the exit temperature of the hot rolling mill is between 370°C and 480°C;(d) further hot rolling from the intermediate gauge in one or more rolling stages to a final hot rolling gauge and in which the exit temperature of the hot rolling mill is between 310°C and 400°C; (e) cooling the hot rolled material at the final hot rolling gauge from the exit temperature of the hot rolling mill to room temperature; (f) cold rolling the hot rolled product to a final gauge cold rolled product.;

[0012] Application US20210340654 discloses a method for producing a 6xxx series aluminum sheet comprising the steps of homogenizing an ingot made of a 6XXX series aluminum alloy comprising in wt% Si: 0.4 to 0.7, Mg: 0.2 to 0.4, Mn: 0.05 to 0.30, Fe: 0.03 to 0.4, Cu up to 0.3, Cr up to 0.05, Zn up to 0.15, Ti up to 0.1 wt%, the remainder being aluminum and unavoidable impurities up to 0.05 each and 0.15 in total, rough hot rolling on a reversing mill to a rough hot rolling exit thickness with a rough hot rolling exit temperature below 420°C, rolling hot finishing the ingot to a final hot rolling thickness with a tandem rolling mill and coiling to the final hot rolling thickness with a hot rolling exit temperature below 300°C, cold rolling to obtain a cold rolled sheet.The products obtained according to the method of the invention are particularly useful for automobile hood interiors because they have the mechanical properties required for pedestrian safety and surface quality. This application aims to provide a sheet for an internal part, such as a hood liner, which is only visible when the hood of the vehicle is open. Such a part is not subject to the same level of surface quality requirements as an external part because there is no need to compromise with, for example, safety in the event of pedestrian impact.

[0013] A new requirement is emerging, which is the waviness of the sheet metal surface in T4 condition. If the sheet metal in T4 condition has too much waviness, a distorting mirror effect appears on the painted body part, which may displease even the most demanding motorists.

[0014] PROBLEM TO BE SOLVED

[0015] The problem to be solved is to develop a 6xxx series alloy sheet which aims for an excellent compromise between:

[0016] • A corrugation less than or equal to 0.54pm of the sheet in state T4,

[0017] • A bending angle of the sheet in state T4 after 6 months of maturation greater than or equal to 125°.

[0018] STATEMENT OF THE INVENTION

[0019] An object of the invention is a method for manufacturing a rolled sheet of 6xxx series aluminum alloy comprising the successive steps: a. Production of a 6xxx series aluminum alloy comprising, in % by weight: i. Fe from 0.10% to 0.40%, ii. Mn from 0.05% to 0.20%, iii. Cr from 0.01% to 0.04%, b. Casting the aluminum alloy into a plate, preferably by semi-continuous vertical casting, c. Homogenization of the plate at a homogenization temperature of 540°C to 580°C, preferably higher than 550°C, followed by cooling, preferably forced, either to a hot rolling start temperature of 400 to 510°C or to a temperature lower than the hot rolling start temperature, d. First hot rolling from the hot rolling start temperature to a first hot rolling end temperature of 370 to 450°C, e.Second hot rolling from the end temperature of the first hot rolling to the end rolling temperature of 250 to 380°C into a strip under conditions such that the microstructure of the strip is recrystallized or not recrystallized after the second hot rolling, f. Cold rolling of the strip, optionally with annealing on a continuous furnace, and without static annealing, g. Solution treatment and then quenching, preferably in air, of the strip into a sheet, h. Preferably, pre-tempering at a pre-tempering temperature of 50 to 120°C for a period of 2 to 16 hours, preferably obtained by coiling and then cooling to room temperature, i. Maturation of 72 hours to 6 months, wherein at least one intermediate recrystallization occurs in steps e and / or f followed by a cold rolling reduction of at least 60%.

[0020] Another object of the invention is a sheet obtained with the method according to the invention.

[0021] Another subject of the invention is a bodywork part of a vehicle, preferably visible, obtained by a process comprising shaping, preferably stamping, then baking the paints of the sheet metal according to the invention.

[0022] DESCRIPTION OF FIGURES

[0023] [Fig. 1]: This figure shows a microstructure of a band in the T4 state.

[0024] [Fig. 2]: This figure shows a microstructure of deformed grains.

[0025] [Fig. 3]: This figure shows a microstructure of restored grains.

[0026] [Fig. 4]: This figure shows a microstructure of elongated recrystallized grains.

[0027] [Fig. 5]: This figure shows a microstructure of large equiaxed recrystallized grains.

[0028] [Fig. 6]: This figure shows a microstructure of small equiaxed recrystallized grains.

[0029] [Fig. 7]: This figure shows a partially recrystallized or mixed microstructure.

[0030] [Fig. 8]: This figure shows a microstructure of a recrystallized strip after hot rolling.

[0031] [Fig. 9]: This figure shows a microstructure of a recrystallized band after annealing on a continuous furnace.

[0032] [Fig. 10]: This figure shows a microstructure of a recrystallized band after static annealing.

[0033] [Fig. 11]: This figure shows examples of samples subjected to lineage characterization, class 1, 2 and 3 (1 average - 3 excellent).

[0034] [Fig. 12]: This figure shows the effect of Na on ripple.

[0035] [Fig. 13]: This figure shows a stamped, painted sheet metal which is a counter-example of the invention.

[0036] [Fig. 14]: This figure shows a stamped, painted sheet metal which is an example of the invention. DETAILED DESCRIPTION OF THE INVENTION

[0037] All aluminum alloys referred to hereinafter are designated, unless otherwise stated, according to the rules and designations defined by the Aluminum Association in the Registration Record Series published by it from time to time. Unless otherwise stated, compositions are expressed in % by weight. The expression 1.4 Cu means that the copper content expressed in % by weight is 1.4%. For Na, compositions are expressed in ppm by weight. The expression 1.4 Na means that the sodium content expressed in ppm by weight is 1.4 ppm.

[0038] Alloy groups, also called series, are defined in EN 573-1 (2005).

[0039] The metallurgical states in question are designated according to European standard EN-515 (2017).

[0040] The static mechanical characteristics in tension, in other words the breaking strength Rm, the conventional yield strength at 0.2% elongation Rp0.2, the elongation at necking Ag% and the elongation at break A%, are determined by a tensile test according to standard NF EN ISO 6892-1 (2018), the sampling and the direction of the test being defined by standard EN 485-1 (2009).

[0041] The bending angles, called alpha norm, are determined by 3-point bending test according to standard NF EN ISO 7438 (2005) and procedures VDA 238-100 version 2010 and VDA 239-200 version 2017.

[0042] Grain sizes are measured using ASTM E112 - 13 (2021).

[0043] Unless otherwise stated, the definitions in EN 12258-1 (2012) apply.

[0044] The lineage is measured as follows. A strip measuring approximately 270 mm (in the transverse direction) by 50 mm (in the rolling direction) is cut from the thin sheet. A tensile pre-strain of 15%, perpendicular to the rolling direction, i.e., along the length of the strip, is then applied. The strip is then subjected to the action of P800 type abrasive paper to reveal the lineage. The latter is then visually assessed and translated by classification on a scale of 1 (significant lineage) to 3 (complete absence of lineage). Examples of lineage corresponding to values ​​1 to 3 are illustrated in [Fig. 11].

[0045] The surface waviness Wsa (1-5) is measured with the SEP1941 standard of May 2012. The Wsa (1-5) waviness measurements are carried out on a sheet in the T4 condition after a deformation of 15% in the cross direction of rolling. The waviness is the average of 14 measurements over a length of 30mm, each measured length is 2.5mm from at least one other measured length.

[0046] Aluminum and aluminum alloys are polycrystalline materials whose characteristics and arrangements can be modified by deformation of the metal (e.g., rolling, extrusion, or forging) or by the application of heat (e.g., annealing). During the deformation of an aluminum alloy, the free energy of the crystalline material can be increased, for example, by crystallographic slip. Crystallographic slip involves the movement of dislocations in certain planes and directions within each crystal. The occurrence of crystallographic slip during plastic deformation increases the dislocation density and crystal rotation in the material. The crystal rotation accompanying deformation is one of the reasons why textures, or non-random crystal orientations, also called grains, develop in a polycrystalline material.Dislocations are therefore imperfections in the crystal of a grain.

[0047] The microstructure of a polycrystalline material, such as an aluminum alloy, varies depending on its thermomechanical history. For example, aluminum alloys may have a deformed microstructure after deformation, a restored microstructure after a restoration anneal, and a recrystallized microstructure after a recrystallization anneal, described in more detail below. An example of a microstructure comprising deformed grains is illustrated in [Fig. 2]. In the illustrated example, the microstructure 2 comprises a plurality of deformed grains 12, each grain having a grain boundary 10. Due to the deformation, the internal areas of the deformed grains 12 comprise a high dislocation density, represented in [Fig. 2] by shading 14.

[0048] To reduce the free energy of a deformed material, it can be annealed. Annealing involves heating the deformed material to a high temperature. There are generally two types of annealing used to treat aluminum alloys: restoration annealing and recrystallization annealing. In restoration annealing, an aluminum alloy is heated to a temperature such that the grain boundary of the deformed grain is generally maintained, but the dislocations within the deformed grains move to lower energy configurations. These lower energy configurations within the grains are called subgrains or cells. Thus, the grains produced by restoration annealing are generally referred to as restored grains. An example of a microstructure comprising restored grains is shown in [Fig. 3]. In the example shown, the restored microstructure 3 comprises 22 restored grains.The restored grains 22 generally have the same grain boundary 10 as the deformed grains 12, but, due to the restoration annealing, sub-grains 16 have formed inside the restored grains 12.

[0049] In recrystallization annealing, the aluminum alloy is heated to a temperature that produces new grains from the deformed grains 12 and / or the restored grains 22. These new grains are called recrystallized grains. Recrystallization annealing results in the production of a material having recrystallized grains. Examples of microstructures comprising recrystallized grains are illustrated in [Fig. 4], [Fig. 5] and [Fig. 6]. In the illustrated examples, microstructure 4 contains elongated recrystallized grains 32c ([Fig. 4]), microstructure 5 contains large equiaxed recrystallized grains 32d ([Fig. 5]), and microstructure 6 contains small equiaxed recrystallized grains 32e ([Fig. 6].). A microstructure is recrystallized when at least 90% of the observed surface is recrystallized. A microstructure is non-recrystallized when at most 10% of the observed surface is recrystallized.

[0050] Under certain circumstances, annealing may produce a partially recrystallized or mixed material, an example of which is illustrated in [Fig. 7]. In the illustrated example, the partially recrystallized or mixed microstructure 7 comprises a mixture of restored grains 22 and recrystallized grains 32.

[0051] Room temperature is any temperature compatible with human work from 5 to 35°C.

[0052] Process

[0053] The invention is based on the applicant's observation that it is entirely possible, using a suitable composition and manufacturing method, to produce sheets with excellent surface quality after painting while retaining excellent bendability. The method is preferably dedicated to sheets for visible bodywork parts, particularly external ones.

[0054] The method for manufacturing a sheet according to the invention comprises the production of an aluminum alloy of the 6xxx series comprising, in % by weight:

[0055] • Fe from 0.10% to 0.40%,

[0056] • Mn from 0.05% to 0.20%,

[0057] • Cr from 0.01% to 0.04%,

[0058] Mn and Cr precipitate during the manufacturing process. They should preferably precipitate as dispersoids, which are small precipitates, with a typical average size of 0.1 to 0.3 pm relative to the grain size. These dispersoids contribute to the control of the various recrystallizations that take place during the manufacturing process and that affect the waviness of the sheet. Fe also precipitates and also contributes to the control of the various recrystallizations. The content of these elements is a compromise that must be in line with the manufacturing process.

[0059] If the Mn and Cr content is excessive, recrystallizations will not occur properly during the manufacturing process, which degrades the waviness. The maximum Mn content is 0.20%; preferably 0.19%, preferably 0.18%, more preferably 0.17%, more preferably 0.16%, more preferably 0.15%. The maximum Cr content is 0.04%, preferably 0.03%, more preferably 0.02%. If the content of these elements is insufficient, the alloy will tend to recrystallize in the form of large grains during solution processing, which also degrades the waviness. The minimum Mn content is 0.05%. The minimum Cr content is 0.01%.

[0060] Too low a Fe content makes the alloy particularly expensive. The addition of Fe, which forms insoluble precipitates, also contributes to the control of recrystallizations but an excess of Fe degrades the bending of the sheet in the T4 state. A high maximum Fe content advantageously makes the alloy tolerant to the use of recycled products during the preparation of the alloy according to the invention. The minimum Fe content is 0.10%, preferably 0.15%, more preferably 0.19%. The maximum Fe content is 0.40%, preferably 0.39%, preferably 0.38%, preferably 0.37%, preferably 0.36%, preferably 0.35%, more preferably 0.34%, more preferably 0.33%, more preferably 0.32%, more preferably 0.31%, more preferably 0.30%.

[0061] According to EN 573-1 (2005), in groups 2xxx to 8xxx, the alloy designation is determined by the addition element (Mg2Si for 6xxx alloys) present in the highest average percentage. If the latter is common to several addition elements, the group will be chosen according to the sequential order Cu, Mn, Si, Mg, Mg2Si, Zn or others. Preferably, the 6xxx series alloy comprises Mg from 0.25% to 1.0% and Si from 0.30% to 1.5%. More preferably, the 6xxx series aluminum alloy further comprises Cu <=0.25%, Zn <=0.25%, Ti <= 0.15%, V <= 0.20%, other elements each <=0.05%, total <=0.15%, remainder AL

[0062] Preferably, the Si is at most 1.05% to improve the surface quality characterized by the lineage.

[0063] In one embodiment, the 6xxx series aluminum alloy further comprises, in wt%:

[0064] • If: 0.30% -1.5%, preferably 0.30%-1.05%, • Cu <=0.25%,

[0065] • Mg 0.25% -0.8%,

[0066] • Zn <= 0.25%,

[0067] • Ti <= 0.15%,

[0068] • optionally V 0.05%-0.20%,

[0069] • other elements each <=0.05%, total <=0.15%,

[0070] • rest Al,

[0071] In one embodiment, the 6xxx series aluminum alloy further comprises, in wt%:

[0072] • Si: 0.5% -1.5%, preferably 0.5%-1.05%, Cu <=0.20%, Mg 0.25% -0.6%, Zn <= 0.20%, Ti

[0073] <= 0.15%, other elements each <=0.05%, total <=0.15%, remainder Al,

[0074] • or Si: 0.6% -0.9%, Cu <=0.10%, Mg 0.40% -0.6%, Zn <= 0.10%, Ti <= 0.10%, other elements each <=0.05%, total <=0.15%, remainder Al,

[0075] • or Si: 0.8% -1.5%, preferably 0.8%-1.05%, Cu 0.01%-0.11%, Mg 0.45% -0.7%, Zn <=

[0076] 0.25%, Ti <= 0.10%, other elements each <=0.05%, total <=0.15%, remainder Al,

[0077] • or Si: 0.30% -0.6%, Cu <=0.25%, Mg 0.40% -0.8%, Zn <= 0.10%, Ti <= 0.10%, V 0.05%-0.20%, other elements each <=0.05%, total <=0.15%, remainder Al,

[0078] • or Si: 0.50% -0.9%, Cu 0.20 -0.8%, Mg 0.40% -0.7%, Zn <= 0.20%, Ti <= 0.10%, V 0.05%-

[0079] 0.20%, other elements each <=0.05%, total <=0.15%, remainder Al.

[0080] Preferably, the Si content is at most 1.05% to improve the lineage.

[0081] Preferably, the 6000 series alloy has the following composition for elements other than Fe, Mn and Cr, in wt%:

[0082] • If: 0.80% -1.05%,

[0083] • Cu: 0.05% - 0.11%,

[0084] • Mg 0.30% -0.50%,

[0085] • optionally Zn <= 0.25%,

[0086] • Ti 0.01% - 0.10%

[0087] • other elements each <=0.05%, total <=0.15%,

[0088] • rest Al,

[0089] Preferably, the Si content of this composition is a maximum of 1.05% to improve the lineage. Si and Mg form Mg2Si precipitates which allow to obtain the mechanical characteristics after the baking of the paints. The excess of Si (subtraction of the Mg content from the Si content) improves the formability in the T4 state.

[0090] Cu contributes to mechanical properties in the T4 and T6 state. Excess Cu can impair corrosion resistance. Preferably, Cu is a maximum of 0.15%, more preferably 0.10%.

[0091] Zn can be added optionally to facilitate recycling but without excess to avoid corrosion.

[0092] Ti plays a role in refining grains. The maximum Ti content is preferably 0.10%.

[0093] In one embodiment of each of the described alloys, the Na content is less than or equal to 2.5 ppm, preferably 2.0 ppm. Controlling the maximum Na content contributes to controlling waviness. A low Na content can be obtained by producing the alloy using very high purity raw material for both the aluminum and the addition elements required for producing the alloy. A less expensive solution is for the production step of the 6xxx series aluminum alloy to include a process for treating the liquid metal to remove all or part of the Na. A non-limiting example of a method for removing all or part of the Na is that taught by application WO2022 / 242992. Preferably, the minimum Na content is 0.3 ppm, more preferably 0.5 ppm. Such a minimum is a relevant compromise between waviness and productivity and cost constraints.

[0094] The aluminum alloy is then cast into a plate, preferably by vertical semi-continuous casting (direct chill casting or DC casting). The preferred dimensions of the plates according to the invention are 200mm to 600mm thick, 1000 to 3000mm wide and 2000 to 8000mm long. Vertical semi-continuous casting makes it possible to obtain a more homogeneous structure of the sheet than that obtained by continuous casting.

[0095] Advantageously, the manufactured sheet metal is monolithic, which is less expensive than a plated sheet metal.

[0096] The plate is then homogenized at a homogenization temperature, between 540°C and 580°C, preferably above 550°C, followed by cooling, preferably forced, either to a hot rolling start temperature of 400 to 520°C, or to a temperature below the hot rolling start temperature. The homogenization temperature is above the solvus temperature of the alloy, while avoiding local melting or burning. The homogenization temperature is preferably a maximum of 580°C, preferably 570°C, and a minimum of 540°C, preferably a minimum of 550°C. Too high or too low a temperature degrades the mechanical properties of the sheet after tempering. Insufficient homogenization in temperature or duration can degrade the undulation by not sufficiently dissolving the Mg2Si, the subsequent precipitates of which will not have the optimal size.The homogenization time is preferably longer than 1 hour. Too short a homogenization time degrades the mechanical properties of the sheet after tempering.

[0097] Homogenization of the plate is followed by cooling, preferably forced, either to a hot rolling start temperature of 400 to 510°C or to a temperature lower than the hot rolling start temperature. Advantageously, this cooling is direct, i.e. without a second intermediate stage during homogenization, to either a hot rolling start temperature of 400 to 510°C or a temperature lower than the hot rolling start temperature so as not to degrade productivity. Cooling the plate after homogenization makes it possible to obtain precipitates of optimal size of Mg2Si which then make it possible to control the recrystallizations necessary in the subsequent stages to obtain the desired waviness.Hot rolling directly at the homogenization temperature for productivity reasons results in Mg2Si precipitates that will cause coarse-grained recrystallizations in later stages that degrade the waviness. Preferably, cooling is forced to avoid excessive growth of Mg2Si precipitates that will deteriorate the solutionization of Mg2Si, and therefore the mechanical properties. Excessive growth of Mg2Si precipitates degrades the waviness.

[0098] In one embodiment, the homogenized plate is directly cooled to the hot rolling start temperature so as not to degrade productivity. This cooling is preferably forced with a direct cooling rate of at least 150°C per hour. Advantageously, the direct cooling rate is at most 500°C / h. This cooling can typically be carried out by a machine such as that described by application WO2016012691. The minimum rate of 150°C is a compromise between precipitation kinetics and productivity. A cooling rate above 500°C / h causes temperature heterogeneities in the plate which can cause precipitation heterogeneities of Mg2Si which can cause a mixed microstructure during subsequent recrystallizations. A mixed microstructure causes inadequate waviness.In another embodiment, the homogenized plate is cooled and then reheated to the hot rolling start temperature. Preferably, this cooling and then reheating is direct to the hot rolling start temperature so as not to degrade productivity. Preferably, this cooling is forced with fans which propel air at ambient temperature onto the homogenized plate to cool it more quickly than naturally to ambient temperature. This cooling preferably takes place to a temperature below 300°C at a preferential rate of 60 to 120°C / h, more preferably 70 to 90°C / h. Preferably, the plate finishes cooling naturally to ambient temperature.Continuing cooling below the hot rolling start temperature is advantageous because it allows the precipitation kinetics of Mg2Si to continue, which promotes recrystallizations during the subsequent stages required for corrugation. The plate is then reheated to the hot rolling start temperature.

[0099] A first hot rolling from the hot rolling start temperature to a first hot rolling end temperature of 370 to 450°C is carried out. This first hot rolling is preferably carried out successively on one rolling mill, two or more hot reversible rolling mills. The end thickness of the first hot rolling is 30 to 50 mm. This first hot rolling is preferably carried out in such a way that the plate does not heat up with each hot rolling pass. Preferably, the cooling between the hot rolling start temperature and the first hot rolling end temperature is a maximum of 90°C, preferably 50°C, more preferably 40°C, more preferably 30°C. Preferably, the difference between the hot rolling start temperature and the first hot rolling end temperature is positive or zero.However, a slight reheating of 10°C, i.e. a difference between the hot rolling start temperature and the first hot rolling end temperature of at least -10°C, during the first hot rolling is acceptable. This hot rolling process allows to control the further precipitation of Mg2Si especially when the plate has been cooled directly to the hot rolling temperature. This first hot rolling temperature range contributes to the control of recrystallizations in the later stages of manufacture. Limiting the maximum value of cooling during the first hot rolling also simplifies the process because it contributes to not needing an intermediate annealing later.

[0100] A second hot rolling is then carried out from the end temperature of the first hot rolling to the end rolling temperature of 250 to 380°C into a strip under conditions such that the microstructure of the strip is recrystallized or not recrystallized after the second hot rolling. That is to say, the microstructure does not include recrystallized zones and not recrystallized zones. A mixed structure after hot rolling will disturb subsequent recrystallizations and degrade the corrugation of the sheet in the T4 state. This second hot rolling is preferably carried out on a tandem hot rolling mill comprising 2, 3, 4, 5, 6 or more hot rolling mills. The strip obtained is then coiled. Preferably, the coil cools naturally to room temperature. Preferably no forced cooling or quenching is carried out during the second hot rolling.Preferably the microstructure is characterized after cooling the strip to room temperature.

[0101] In one embodiment, the end of hot rolling temperature is greater than 330°C, preferably greater than 340°C, more preferably greater than 350°C. A high temperature at the end of hot rolling makes it possible to obtain a medium-grain recrystallization which is favorable for the final recrystallization to obtain a waviness of less than 0.50 pm. This high temperature makes it possible to simplify the manufacturing process by not requiring an intermediate annealing.

[0102] In another embodiment, the end of hot rolling temperature is less than 330°C, preferably less than 325°C, more preferably less than 320°C. A low temperature at the end of hot rolling makes it possible to obtain a non-recrystallized microstructure which is favorable for obtaining subsequent recrystallizations to obtain a waviness of less than 0.50 pm.

[0103] The strip is then cold rolled, optionally with an intermediate annealing in a continuous furnace, and without static annealing. Annealing is intermediate when the intermediate annealing is preceded and followed by cold rolling. Static annealing is carried out in a furnace in which the coiled strip undergoes an annealing heat treatment. No static annealing is carried out before, during or after the cold rolling step. Static annealing does not allow the compromise between surface quality or bendability to be achieved because the duration of this heat treatment results in coarse grains.

[0104] Preferably, the total cold reduction is at least 75%. The total cold reduction is the reduction between the thickness at the end of hot rolling and the final thickness. This reduction contributes to obtaining a waviness less than or equal to 0.50 pm.

[0105] Preferably, the annealing in a continuous furnace is a recrystallization annealing. In one embodiment, the annealing in a continuous furnace is carried out with a PMT (peak metal temperature) lower than the solvus temperature of the alloy. In another embodiment, the annealing in a continuous furnace also carries out a solution treatment with a temperature higher than the solvus and lower than the burn temperature. Preferably, the PMT is chosen high to minimize the time above 350°C in order to obtain a medium-grain recrystallization. A recrystallization in a continuous furnace means that cold rolling takes place before the solution treatment.

[0106] The thickness of the strip after cold rolling is 0.8 to 1.2 mm. Too thin a thickness is not usable because the body part does not have sufficient rigidity. Too thick a thickness makes the body part too heavy for its use.

[0107] At least one intermediate recrystallization occurs in the second hot rolling and / or cold rolling stages followed by a cold rolling reduction of at least 60%. Recrystallization occurred during the second hot rolling stage when the microstructure of the strip after hot rolling is recrystallized. Recrystallization in the cold rolling stage is preferably achieved with annealing on a continuous furnace. Intermediate recrystallization is followed by cold rolling whose reduction of at least 60% deforms the recrystallized grains in order to obtain a fine-grained recrystallization during the solution treatment necessary for corrugation. This reduction of at least 60% is the reduction between the intermediate recrystallization thickness and the final thickness.

[0108] The absence of intermediate recrystallization according to the invention causes either insufficient folding or a surface quality characterized by unsuitable alignment for visible bodywork parts. The intermediate recrystallization according to the invention combined with cold rolling between the intermediate recrystallization and the solution treatment makes it possible to control the final recrystallization which takes place during the solution treatment and to obtain the desired undulation. Preferably, the average grains obtained by this intermediate recrystallization are elongated grains with a length dimension in the long rolling direction of 70 to 200 μm as shown in [Fig. 9]. The ratio of the length to the thickness of the recrystallized grains is 2 to 5. The length is measured along the long rolling direction. The thickness is measured along the short cross direction.The recrystallized grains obtained after hot rolling have this dimension at mid-thickness as shown in [Fig. 8]. Too long grains leave a legacy to the grains resulting from the final recrystallization carried out with solution treatment, this legacy will cause inappropriate waviness. This is the case of grains obtained by recrystallization with static annealing as shown in [Fig. II].

[0109] The reduction in cold rolling between intermediate recrystallization and solution treatment is at least 60% in order to deform the strip sufficiently to obtain the final recrystallization during solution treatment necessary to obtain the corrugation.

[0110] The sheet is then solution-treated at a solution temperature above the solvus temperature of the alloy, while avoiding local melting or burning, and then quenched, preferably in a continuous furnace. The solution-treated temperature is preferably a maximum of 580°C, preferably 570°C, and a minimum of 540°C, preferably a minimum of 550°C. Too cold a solution-treated temperature and / or too short a solution-treated temperature degrade the mechanical properties of the sheet by insufficient solution-treated temperature. Too hot a solution-treated temperature causes burning, which degrades the mechanical properties. Too long a solution-treated temperature degrades productivity.

[0111] Solution treatment also causes a final recrystallization. Preferably, this is a fine-grained recrystallization with an average length of 10 to 50 pm as shown in [Fig. 10]. Preferably, the recrystallized grains have a length to thickness ratio of at most 2 as shown in [Fig. 10]. The grains are measured in length along the rolling direction and in thickness along the short cross direction. Larger or more elongated grains deteriorate the waviness. This microstructure is not modified by pre-tempering and maturation.

[0112] The solution temperature is preferably at least 540°C, preferably 550°C, and at most 570°C. Preferably, there is no maintenance at the solution temperature to avoid coarsening of the grains.

[0113] Hardening is preferably carried out in air to limit flatness defects.

[0114] Preferably, the sheet is pre-tempered. Preferably, the sheet is reheated to achieve pre-tempering at a pre-tempering temperature of 50°C to 120°C, preferably 65 to 90°C for a period of 2 to 16 hours. Reheating is useful when the sheet undergoes a surface treatment between quenching and pre-tempering whose temperature is lower than pre-tempering. Preferably, pre-tempering is obtained by reheating without maintaining the pre-tempering temperature, then coiling, then cooling to room temperature, preferably for at least 40 hours. Pre-tempering stabilizes natural aging and improves the response to paint baking, which is the difference between the yield strength in the T4 state and the yield strength after paint baking.

[0115] The sheet metal matures to the T4 state at room temperature between 72 hours and 6 months. This stage is a constraint linked to storage before forming. The sheet metal is then in the T4 state.

[0116] The sheet according to the invention can be obtained by the method according to the invention. Preferably, the sheet according to the invention is characterized by the average dimension of the recrystallized grains being 10 to 50 μm in length, preferably 20 μm to 40 μm and / or by the ratio of the length in the long direction to the thickness in the short cross direction of the recrystallized grains being less than or equal to 2.

[0117] Preferably, the sheet metal according to the invention is characterized by a waviness of less than 0.54 pm, preferably 0.50 pm, more preferably 0.48 pm. Reducing the waviness improves the quality of the surface after painting.

[0118] Preferably, the sheet according to the invention obtains an elastic limit, in the cross rolling direction, after simulation of the baking of the paints (bake hardening) of at least 160 MPa, preferably at least 170 MPa, more preferably at least 180 MPa, more preferably at least 190 MPa, more preferably at least 195 MPa. Too low an elastic limit makes the part sensitive to indentation, that is to say to the impact of hailstones. Preferably, the elastic limit is less than 260 MPa, preferably 250 MPa, more preferably 240 MPa, more preferably 230 MPa. Too high an elastic limit degrades the bendability of the part in the event of an accident. The simulation of the baking of the paints is carried out after a pre-tension of 2%, in the cross direction during rolling, then a heat treatment of 20 minutes at 185°C.

[0119] A body part, preferably visible, of a vehicle can be produced with the sheet metal according to the invention. The production process comprises shaping, preferably by stamping, then baking the paints. Baking the paints, known to those skilled in the art, corresponds to a heat treatment of 10 to 30 minutes at a temperature between 170 and 195°C.

[0120] Embodiment 1

[0121] In a first embodiment, the homogenized plate is directly cooled to the hot rolling start temperature as described above.

[0122] The hot rolling start temperature is 450 to 520°C, preferably 470 to 510°C, more preferably 470 to 400°C. The hot rolling end temperature is below 330°C, preferably below 325°C, more preferably below 320°C. Too high a temperature is likely to cause recrystallization. Too low a temperature induces excessive hot rolling stresses. This temperature range makes it possible to obtain a non-recrystallized microstructure of the strip after hot rolling which recrystallizes to medium grains during the intermediate annealing to obtain the corrugation. An intermediate annealing in a continuous furnace is carried out during the cold rolling step. This intermediate annealing is a recrystallization according to the description above.This combination of a non-recrystallized structure after hot rolling with annealing in a continuous furnace allows the production of finer grains than with a structure recrystallized after hot rolling.

[0123] Embodiment 2

[0124] In a second embodiment, the homogenized plate is cooled and then reheated to the hot rolling start temperature as described above.

[0125] The hot rolling start temperature is below 450°C, preferably 440°C, more preferably 430°C, more preferably 420°C. Limiting the hot rolling start temperature allows for reduced cooling during the first hot rolling. The hot rolling end temperature is above 330°C, preferably above 340°C, more preferably above 350°C. The microstructure of the strip is recrystallized to medium grains after hot rolling. A higher temperature at the end of hot rolling improves intermediate recrystallization and then final recrystallization to obtain a waviness below 0.50 pm.

[0126] Stage g preferably does not involve any intermediate annealing in a continuous furnace, which is advantageous for productivity, especially since it is often expensive equipment and requires a size that is appropriate for the production capacity of the factories that have it.

[0127] Cold reduction is preferably greater than 75%, preferably greater than 80% in order to improve recrystallization and obtain better waviness. Cold reduction is the reduction between the end thickness of hot rolling and the final thickness.

[0128] Embodiment 3

[0129] In a third embodiment, the homogenized plate is directly cooled to the hot rolling start temperature as described above.

[0130] The hot rolling start temperature is below 450°C, preferably 440°C, more preferably 430°C, more preferably 420°C. Limiting the hot rolling start temperature allows for reduced cooling during the first hot rolling. The hot rolling end temperature is above 330°C, preferably above 340°C, more preferably above 350°C. The microstructure of the strip is recrystallized to medium grains after hot rolling. A higher temperature at the end of hot rolling improves intermediate recrystallization and then final recrystallization to obtain a waviness below 0.50 pm.

[0131] Stage g preferably does not involve any intermediate annealing in a continuous furnace, which is advantageous for productivity, especially since it is often expensive equipment and requires a size that is appropriate for the production capacity of the factories that have it.

[0132] The cold reduction is preferably greater than 75%, preferably greater than 80% in order to improve recrystallization and obtain better waviness.

[0133] EXAMPLES

[0134] The disclosure is further illustrated by the following examples. These examples are intended only to illustrate the invention and not to limit it.

[0135] Plates of different compositions were cast according to the alloys in Table 1. These plates were cast by semi-continuous vertical casting. The examples according to the invention correspond to plates A, B, C, D, E, F, M, N, O and P.

[0136] [Table 1] These plates were homogenized at a temperature of 560°C for 3 hours except for plates D and E which were homogenized for 12 hours at a temperature of 560°C. Plates D and E were cooled with fans to 300°C at a cooling rate of 87°C / h and then these plates cooled to room temperature and were reheated to the hot rolling start temperature. The other plates were cooled directly to the hot rolling start temperature with a cooling rate of 150 to 500°C / h.

[0137] A first hot rolling is then carried out on a reversible hot rolling mill. The temperatures at the start of hot rolling and at the end of the first hot rolling are given in Table 2. The thickness after this first hot rolling is between 30 and 50 mm.

[0138] A second hot rolling is then carried out on a tandem rolling mill with 4 rolling mills. The end temperature of hot rolling is given in Table 2. The coiled strips were then cooled naturally to room temperature. The microstructures were then analyzed. The strips after hot rolling A, B, C, D, E, H, M, N, O and P are recrystallized. The grains are elongated in length in the rolling direction from 70 to 200 μm. The ratio of the length to the thickness of the recrystallized grains is 2 to 5. The strips F, G, I, J, K and L are not recrystallized after hot rolling.

[0139] [Table 2]

[0140] The strips were then cold rolled according to Table 3, which also specifies the strips that underwent intermediate recrystallization annealing. The microstructures of the strips that underwent intermediate recrystallization annealing were analyzed. The F strip recrystallized after intermediate annealing. The grains of the F strip are elongated in length in the rolling direction from 70 to 200 pm. The ratio of the length to the thickness of the recrystallized grains is 2 to 5. The I, K and L strips recrystallized with grains more than 200 pm.

[0141] [Table 3] The strips were then solution-treated at a temperature of 560°C (PMT). Strips A, B, C, D, E, F, M, N, O and P recrystallized to fine grains with an average size of 10 to 50 pm and a length to thickness ratio of at most 2. The sheets were then pre-tempered and then matured for 180 days and are in the T4 state. The sheets thus obtained were characterized and the results are in Table 4. The TT bending corresponds to the QQ direction in the cited standards. The yield strength was measured after a 2% pre-tension, in the transverse direction during rolling, and a 20-minute heat treatment at 185°C to simulate the effect of paint baking.

[0142] [Table 4]

[0143] The manufacturing process makes it possible to obtain sheets with the desired properties.

[0144] Figures 13 and 14 show the effect of the ripple observable by the most demanding motorists. Both figures show the reflected image of a lamp by a stamped sheet metal, painted with black paint in a manner comparable to that done in an automobile factory. Figure 13 is obtained with a sheet metal which is a counter-example of the invention with a ripple greater than 0.54 pm. Figure 14 is an example of the invention with a ripple less than 0.48 pm. In Figure 13, the contours of the reflected image of the lamp are blurred by the effect of the ripples which have a distorting mirror effect. On the contrary, in Figure 13, the contours of the reflected image of the lamp are much sharper.

Claims

CLAIMS 1. A method of manufacturing a sheet comprising the successive steps: a. Production of an aluminum alloy of the 6xxx series comprising, in % by weight: i. Fe from 0.10% to 0.40%, ii. Mn from 0.05% to 0.20%, iii. Cr from 0.01% to 0.04%, b. Casting the aluminum alloy into a plate, preferably by semi-continuous vertical casting, c. Homogenization of the plate at a homogenization temperature, between 540°C and 580°C, preferably higher than 550°C, followed by cooling, preferably forced, either to a hot rolling start temperature of 400 to 520°C, or to a temperature lower than the hot rolling start temperature, d. First hot rolling from the start temperature of hot rolling to a first hot rolling end temperature of 370 to 450°C, e.Second hot rolling from the end temperature of the first hot rolling to the end rolling temperature of 250 to 380°C into a strip under conditions such that the microstructure of the strip is recrystallized or not recrystallized after the second hot rolling, f. Cold rolling of the strip, optionally with annealing on a continuous furnace, and without static annealing, g. Solution treatment and then quenching, preferably in air, of the strip into a sheet, h. Preferably, pre-tempering at a pre-tempering temperature of 50 to 120°C for a period of 2 to 16 hours, preferably obtained by coiling and then cooling to room temperature, i. Maturation of 72 hours to 6 months, wherein at least one intermediate recrystallization occurs in steps e and / or f followed by a reduction by cold rolling of at least 60%. 2) Method according to claim 1 characterized in that the aluminum alloy of the 6xxx series further comprises, in % by weight: • Mg: 0.25% - 1.0%, • If: 0.30% - 1.5%, • preferably Cu <=0.25%, • preferably Zn <=0.25%, • preferably Ti <= 0.15%, • preferably V <= 0.20%, • preferably other elements each <=0.05%, total <=0.15%, • rest Al. 3) Method according to claim 1 characterized in that the aluminum alloy of the 6xxx series further comprises, in % by weight: • If: 0.30% -1.5%, preferably 0.30%-1.05%, • Cu <=0.25%, • Mg 0.25% -0.8%, • Zn <= 0.25%, • Ti <= 0.15%, • optionally V 0.05%-0.20%, • other elements each <=0.05% maximum, total <=0.15%, • rest Al. 4) Method according to claim 1 characterized in that the alloy of the 6xxx series further comprises, in % by weight: • Si: 0.5% -1.5%, preferably 0.5%-1.05%, Cu <=0.20%, Mg 0.25% -0.6%, Zn <= 0.20%, Ti <= 0.15%, other elements each <=0.05%, total <=0.15%, remainder Al, • or Si: 0.6% -0.9%, Cu <=0.10%, Mg 0.40% -0.6%, Zn <= 0.10%, Ti <= 0.10%, other elements each <=0.05%, total <=0.15%, remainder Al, • or Si: 0.8% -1.5%, preferably 0.8%-1.05%, Cu 0.01%-0.11%, Mg 0.45% -0.7%, Zn <= 0.25%, Ti <= 0.10%, other elements each <=0.05%, total <=0.15%, remainder Al, • or Si: 0.30% -0.6%, Cu <=0.25%, Mg 0.40% -0.8%, Zn <= 0.10%, Ti <= 0.10%, V 0.05%- 0.20%, other elements each <=0.05%, total <=0.15%, remainder Al, • or Si: 0.50% -0.9%, Cu 0.20 -0.8%, Mg 0.40% -0.7%, Zn <= 0.20%, Ti <= 0.10%, V 0.05%- 0.20%, other elements each <=0.05%, total <=0.15%, remainder Al. 5) Method according to one of the preceding claims, characterized in that the Si content of these compositions is at most 1.05% by weight. 6) Method according to claim 1 characterized in that the alloy of the 6xxx series further comprises, in % by weight: • If: 0.80% -1.05%, • Cu: 0.05 - 0.20%, • Mg 0.30% -0.50%, • optionally Zn <= 0.25%, • Ti 0.01% -0.10%, • other elements each <=0.05% max, total <=0.15%, • rest Al. 7) Method according to one of the preceding claims, characterized in that the alloy of the 6xxx series comprises a Na content <=2.5 ppm by weight, preferably <=2.0 ppm. 8) Method according to one of the preceding claims, characterized in that the production step a comprises a method of treating the liquid metal making it possible to remove all or part of the Na. 9) Method according to one of the preceding claims, characterized in that the cooling of step c is directly down to said temperatures. 10) Method according to one of the preceding claims, characterized in that the difference between the start temperature of hot rolling and the end temperature of the first hot rolling is at least -10°C and at most 90°C, preferably 50°C, more preferably 40°C, more preferably 30°C. 11) Method according to one of the preceding claims, characterized in that the total cold reduction is at least 75%. 12) Method according to one of the preceding claims, characterized in that • step d is preferably a cooling directly from the homogenization temperature to the rolling start temperature, • the hot rolling start temperature is 450 to 520°C, • the end of hot rolling temperature is less than 330°C, preferably less than 325°C, • the microstructure of the band after step e is not recrystallized, • step g includes an intermediate annealing in a continuous furnace which carries out recrystallization of the strip. 13) Method according to one of claims 1 to 11, characterized in that • step d is cooling to a temperature below the hot rolling start temperature which is below 450°C, • the end temperature of the second hot rolling is greater than 340°C, preferably greater than 350°C, • the microstructure of the strip after step e is recrystallized, step g preferably does not include any intermediate annealing. 14) Method according to one of claims 1 to 11, characterized in that • step d is a direct cooling from the homogenization temperature to the hot rolling start temperature which is less than 450°C, • the end temperature of the second hot rolling is greater than 340°C, preferably greater than 350°C, • the microstructure of the strip after step e is recrystallized, • step g preferably does not include any intermediate annealing. 15) Sheet in T4 condition obtained by the process according to any one of claims 1 to 9. 16) Sheet according to claim 15 characterized in that the average dimension of the recrystallized grains, measured with the ASTM E112 - 13 standard, is 10 to 50 pm in length, preferably 20 pm to 40 pm and / or the ratio of the length in the long direction to the thickness in the short transverse direction of the recrystallized grains is less than or equal to 2. 17) Sheet according to claim 15 or 16 characterized in that the sheet obtains, after a pre-tension of 2%, in its rolling crosswise direction, then a heat treatment of 20 minutes at 185°C, an elastic limit, in the rolling crosswise direction, of at least 160 MPa and / or at most 260 MPa. 18) Sheet according to one of claims 15 to 17 characterized in that the undulation, measured with the SEP1941 standard of May 2012, is less than 0.54pm, preferably less than 0.50pm, more preferably less than 0.48pm. 19) Bodywork part of a vehicle, preferably visible, obtained by a process comprising shaping, preferably stamping, then baking the paints of the sheet metal according to one of claims 15 to 18.

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