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 improved quality after painting.

Description

TECHNICAL FIELD

[0001] The invention relates to the field of aluminum alloy sheets intended for manufacturing, by stamping, bodywork parts of the body-in-white of automobiles.PRIOR ART

[0002] Aluminum alloys are increasingly being used in automobile construction for reducing the weight of vehicles and thus reducing fuel consumption and greenhouse gas emissions.

[0003] Aluminum alloys are used in particular for producing bodywork parts and in particular visible bodywork parts, more particularly external ones. Visible bodywork parts, more particular external ones, are subject to technical specifications so that these parts, after painting, have the required appearance to satisfy automobile manufacturers. Conventionally, the quality of the surface is characterized by measuring the roping but this characterization does not reveal all the surface defects.

[0004] The application EP1967598 discloses a metal sheet made from a type 6000 aluminum alloy containing Si and Mg as main alloy components and having excellent suitability for forming sufficient to allow bending when flat, excellent resistance to denting, and good suitability for hardening during paint curing. The application discloses a method for producing an aluminum alloy sheet that consists of subjecting an ingot to homogenization treatment, to cooling to a temperature below 350° C. at a cooling rate of 100° C. / h or more, optionally to ambient temperature, to heating once again to a temperature of 300 to 500° C. and subjecting it to hot rolling, to cold rolling the hot-rolled products, and to subjecting the cold-rolled sheet to solution heat treatment at a temperature of 400° C. or more, followed by quenching.

[0005] The application EP3485055 discloses a method for producing a 6xxx series aluminum sheet comprising the steps of homogenizing a 6xxx series aluminum ingot; cooling the homogenized ingot at a cooling rate of between 150° C. / h and 2000° C. / h directly to a hot-rolling start temperature; hot rolling the ingot to a final hot-rolling thickness and coiling at the final hot-rolling thickness with conditions such that at least 50% crystallization is obtained; cold rolling to obtain a cold-rolled sheet. The method according to the invention is particularly useful for manufacturing metal sheets for the automobile industry that 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.

[0006] The application WO2018 / 206696 discloses a method for manufacturing a rolled sheet of aluminum alloy having excellent formability and good suitability for hardening by paint curing, comprising: (a) casting an ingot of an Al—Si—Mg aluminum alloy comprising, as % by weight, 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 approximately 4 hours; cooling the ingot to a temperature of between 460° C. and 520° C.; and maintaining the ingot at a temperature between 460° C. and 520° C. for less than 6 hours; (e) hot rolling the ingot in one or more rolling steps to an intermediate size of between 15 mm and 40 mm and wherein the exit temperature from the hot rolling mill is between 370° C. and 480° C.; (d) further hot rolling from the intermediate gauge in one or more rolling steps to a final hot-rolling gauge and wherein the exit temperature from the hot rolling mill is between 310° C. and 400° C.; (e) cooling the hot-rolled material to the final hot-rolling gauge from the exit temperature from the hot rolling mill to ambient temperature; (f) cold rolling the hot-rolled product to a cold-rolled product with the final gauge.

[0007] The application US20210340654 discloses a method for producing a 6xxx series aluminum sheet comprising the steps consisting in homogenizing an ingot consisting of a 6xxx series aluminum alloy comprising, as % by weight 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% by weight, the remainder being aluminum and unavoidable impurities up to 0.05 each and 0.15 in total, with hot rolling on a reversible rolling mill to a thickness on exiting the hot rolling with at a rough hot rolling exit temperature below 420° C., finishing hot rolling of the ingot to a final hot-rolling thickness with a tandem rolling mill and coiling at the final hot-rolling thickness with a hot-rolling exit temperature below 300° C., and 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 since they have the required mechanical properties for the safety of pedestrians and surface quality. This application aims to provide a metal sheet for an internal part, such as a hood lining, that is visible only when the vehicle hood is open. Such a part is not subject to the same level of surface-quality requirement as an external part since it is not necessary to establish a compromise with, for example, safety in the case of a pedestrian impact.

[0008] A new requirement emerges, which is the waviness of the surface of the metal sheet in T4 state. If the sheet in T4 state has too much waviness, a deforming mirror effect liable to displease the most demanding motorists appears on the painted bodywork part.Problem to be Solved

[0009] The problem to be solved is to develop a 6xxx series alloy sheet that aims for an outstanding tradeoff between:

[0010] A waviness of less than or equal to 0.54 μm of the sheet in T4 state,

[0011] A bending angle of the sheet in T4 state after 6 months of natural aging greater than or equal to 125°.DISCLOSURE OF THE INVENTION

[0012] One object of the invention is a method for manufacturing a rolled sheet made from 6xxx series aluminum alloy comprising the following successive steps:

[0013] a. Producing a 6xxx series aluminum alloy comprising, as % by weight:

[0014] i. Fe from 0.10% to 0.40%,

[0015] ii. Mn from 0.05% to 0.20%,

[0016] iii. Cr from 0.01% to 0.04%,

[0017] b. Casting of the aluminum alloy in a plate, preferentially by semicontinuous vertical casting,

[0018] c. homogenization of the plate at a homogenization temperature, between 540° C. and 580° C., preferentially above 550° C., followed by cooling, preferentially forced, either to a hot-rolling start temperature of 400 to 510° C. or to a temperature below the hot-rolling start temperature,

[0019] d. First hot rolling from the hot-rolling start temperature to a temperature of end of the first hot rolling of 370 to 450° C.,

[0020] e. Second hot rolling from the end temperature of first hot rolling to the end of rolling temperature of 250 to 380° C. into a strip under conditions such that the microstructure of the strip is recrystallized or non-recrystallized after the second hot rolling,

[0021] f. Cold rolling of the strip, with optionally with annealing on a continuous furnace, and without static annealing,

[0022] g. Solution heat treatment, preferentially in air, of the strip into a sheet,

[0023] h. Preferably, pre-aging at a pre-aging temperature of 50 to 120° C. for a period of 2 to 16 hours, preferentially obtained by coiling and then cooling to ambient temperature,

[0024] i. Natural aging from 72 hours to 6 months,

[0025] wherein at least an intermediate recrystallization occurs in steps e and / or f followed by a reduction by cold rolling of at least 60%.

[0026] Another object of the invention is a metal sheet obtained with the method according to the invention.

[0027] Another object of the invention is a vehicle bodywork part, preferentially visible, obtained by a method comprising the formation, preferentially pressing, and then curing of the paint on the sheet according to the invention.DESCRIPTION OF THE FIGURES

[0028] FIG. 1: This figure shows a microstructure of a strip in T4 state.

[0029] FIG. 2: This figure shows a microstructure of deformed grains.

[0030] FIG. 3: This figure shows a microstructure of restored grains.

[0031] FIG. 4: This figure shows a microstructure of elongated recrystallized grains.

[0032] FIG. 5: This figure shows a microstructure of large equiaxial recrystallized grains.

[0033] FIG. 6: This figure shows a microstructure of small equiaxial recrystallized grains.

[0034] FIG. 7: This figure shows a partially recrystallized or mixed microstructure.

[0035] FIG. 8: This figure shows a microstructure of a recrystallized strip after hot rolling.

[0036] FIG. 9: This figure shows a microstructure of a recrystallized strip after annealing on a continuous furnace.

[0037] FIG. 10: This figure shows a microstructure of a recrystallized strip after static annealing.

[0038] FIG. 11: This figure shows examples of samples subjected to looper-line characterization, of class 1, 2 and 3 (1 average-3 excellent).

[0039] FIG. 12: This figure shows the effect of Na on the waviness.

[0040] FIG. 13: This figure shows a sheet, pressed and painted, which is a counter-example of the invention.

[0041] FIG. 14: This figure shows a sheet, pressed and painted, which is an example of the invention.DETAILED DESCRIPTION OF THE INVENTION

[0042] All the aluminum alloys in question hereinafter are, unless specified otherwise, designated according to the rules and designations defined by the “Aluminum Association” in the “Registration Record Series” that it publishes regularly. Unless specified otherwise, the compositions are expressed as % by weight. The expression 1.4 Cu means that the copper content expressed as % by weight is 1.4%. For Na, the 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.

[0043] The groups of alloys, also called series, are defined in EN 573-1 (2005).

[0044] The metallurgical states in question are designated in accordance with the European standard EN 515 (2017).

[0045] The static tensile mechanical properties, in other words the ultimate tensile strength Rm, the conventional yield strength at 0.2% elongation Rp0.2, the striction elongation Ag % and the elongation at rupture A % are determined by a tensile test according to NF EN ISO 6892-1 (2018), the sampling and the direction of the test being defined by EN 485-1 (2009).

[0046] The bending angles, called alpha norm, are determined by 3-point bending test according to NF EN ISO 7438 (2005) and the VDA 238-100 version 2010 and VDA 239-200 version 2017 procedures.

[0047] The grain sizes are measured in accordance with ASTM E112-13 (2021).

[0048] Unless specified otherwise, the definitions of EN 12258-1 (2012) apply.

[0049] Looper lines are measured in the following way. A strip measuring about 270 mm (in the transverse direction) by 50 mm (in the rolling direction) is cut into the sheet. A traction pre-deformation of 15%, perpendicular to the rolling direction, that is to say in the direction of the length of the strip, is subsequently applied. The strip is subsequently subjected to the action of an abrasive paper of the P800 type in order to show the looper lines. The latter is then visually evaluated and translated by classification on a scale of 1 (significant looper lines) to 3 (total absence of looper lines). Examples of looper lines corresponding to the values 1 to 3 are illustrated in [FIG. 11].

[0050] The waviness Wsa (1-5) of the surface is measured with the standard SEP1941 of May 2012. The measurements of waviness Wsa (1-5) are made on a sheet in state T4 after a deformation of 15% in the transverse rolling direction. Waviness is the average of 14 measurements over a length of 30 mm, each measured length is at least 2.5 mm from another measured length.

[0051] Aluminum and aluminum alloys are polycrystalline materials, the characteristics and arrangements of which can be modified by deformation of the metal (for example rolling, extrusion or forging) or by applying heat (e.g. annealing). During the deformation of an aluminum alloy, the free energy of the crystalline material may be increased, for example by crystallographic slip. Crystallographic slip involves the movement of the dislocations in some planes and directions in each crystal. The appearance of crystallographic slip during plastic deformation increases the density of the dislocations and the rotation of the crystals in the material. The rotation of the crystals accompanying the deformation is one of the reasons why the textures, or the non-random orientations of the crystals, also referred to as grains, develop in a polycrystalline material. Dislocations are therefore imperfections of the crystal of a grain.

[0052] The microstructure of a polycrystalline material, such as an aluminum alloy, varies according to the thermomechanical history thereof. For example, aluminum alloys may have a deformed structure after deformation, a restored microstructure after restoration annealing, and a recrystallized microstructure after a recrystallization annealing, described in more detail below. An example of a microstructure comprising deformed grains is illustrated on [FIG. 2]. In the example illustrated, the microstructure 2 comprises a plurality of deformed grains 12, each grain having a grain boundary 10. Because of the deformation, the internal regions of the deformed grains 12 comprise a high dislocation density, shown on [FIG. 2] by shading 14.

[0053] To reduce the free energy of a deformed material, the latter can be annealed. Annealing consists in heating the deformed material to a high temperature. There generally exist two types of annealing used for treating aluminum alloys: restoration annealings and recrystallization annealings. In the case of a 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 inside the deformed grains move towards configurations with a low energy. These lower-energy configurations inside the grains are referred to as sub-grains or cells. Thus the grains produced by a restoration annealing are generally referred to as restored grains. An example of a microstructure comprising restored grains is illustrated on [FIG. 3]. In the example illustrated, the restored microstructure 3 comprises restored grains 22. The restored grains 22 generally have the same grain boundary 10 as the deformed grains 12 but, because of the restoration annealing, sub-grains 16 have formed inside the restored grains 12.

[0054] During a recrystallization annealing, the aluminum alloy is heated to a temperature that produces new grains from the deformed grains 12 and / or from the restored grains 22. These new grains are referred to as recrystallized grains. A 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 examples illustrated, the microstructure 4 contains elongated recrystallized grains 32c ([FIG. 4]), the microstructure 5 contains large equiaxial recrystallized grains 32d ([FIG. 5]), and the microstructure 6 contains small equiaxial 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 no more than 10% of the observed surface is recrystallized.

[0055] In some circumstances, annealing may produce a partially recrystallized or mixed material, an example of which is illustrated in [FIG. 7]. In the example illustrated, the partially recrystallized or mixed microstructure 7 comprises a mixture of restored grains 22 and recrystallized grains 32.

[0056] Ambient temperature is any temperature compatible with the work of humans from 5 to 35° C.Method

[0057] The invention is based on the observation made by the applicant that it is absolutely possible, thanks to a suitable composition and manufacturing process, to produce sheets with excellent surface quality after painting while keeping excellent suitability for bending. The method is preferably dedicated to sheets for visible bodywork parts, in particular external.

[0058] The method for manufacturing a sheet according to the invention comprises producing a 6xxx series aluminum alloy comprising, as % by weight:

[0059] Fe from 0.10% to 0.40%,

[0060] Mn from 0.05% to 0.20%,

[0061] Cr from 0.01% to 0.04%,

[0062] The Mn and Cr precipitate during the manufacturing method. They must preferably precipitate in the form of dispersoids, which are small precipitates, with a typical mean dimension of 0.1 to 0.3 μm with respect to the size of the grains. These dispersoids contribute to controlling the various recrystallizations that take place during the manufacturing method and influence the waviness of the sheet. Fe also precipitates and also contributes to controlling the various recrystallizations. The content of these elements is a compromise that must match the manufacturing method.

[0063] In the case of an excess of Mn and Cr content, the recrystallizations will not take place correctly during the manufacturing method, which degrades the waviness. The maximum proportion of Mn is 0.20%; preferably 0.19%, preferably 0.18%, more preferably 0.17%, more preferably 0.16%, more preferably 0.15%. The maximum proportion of Cr is 0.04%, preferably 0.03%, more preferably 0.02%. In the case of an insufficient proportion of these elements, the alloy will have a tendency to crystallize in the form of large grains during the solution heat treatment, which also degrades the waviness. The minimum proportion of Mn is 0.05%. The minimum proportion of Cr is 0.01%.

[0064] An excessively low proportion of Fe makes the alloy particularly expensive. Adding Fe, which forms insoluble precipitates, also helps to control the recrystallizations but an excess of Fe degrades the bending of the sheet in T4 state. A high maximum proportion of Fe advantageously makes the alloy tolerant to the use of recycled products during the production of the alloy according to the invention. The minimum Fe is 0.10%, preferentially 0.15%, more preferentially 0.19%. The maximum Fe 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%.

[0065] According to EN 573-1 (2005), in the groups 2xxx to 8xxx, the designation of the alloy is determined by the addition element (Mg2Si for 6xxx alloys) present as the highest mean percentage. If the latter is common to several addition elements, the group will be selected in the sequential order Cu, Mn, Si, Mg, Mg2Si, Zn or others. Preferably, the alloy in the 6xxx series comprises Mg from 0.25% to 1.0% and Si from 0.30% to 1.5%. More preferably, the aluminum alloy in the 6xxx series furthermore comprises Cu<=0.25%, Zn<=0.25%, Ti<=0.15%, V<=0.20%, other elements each <=0.05%, total <=0.15%, remainder Al.

[0066] Preferably, the Si is at a maximum 1.05% to improve the surface quality characterized by looper lines.

[0067] In one embodiment, the aluminum alloy in the 6xxx series furthermore comprises, as % by weight:

[0068] Si: 0.30%-1.5%, preferably 0.30%-1.05%

[0069] Cu<=0.25%,

[0070] Mg 0.25%-0.8%,

[0071] Zn<=0.25%,

[0072] Ti<=0.15%,

[0073] optionally V 0.05%-0.20%,

[0074] other elements each <=0.05%, total <=0.15%,

[0075] remainder Al

[0076] In one embodiment, the aluminum alloy in the 6xxx series furthermore comprises, as % by weight:

[0077] 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,

[0078] 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,

[0079] 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,

[0080] 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,

[0081] 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.

[0082] Preferably, the Si content is at a maximum 1.05% to improve the looper lines.

[0083] Preferably, the 6000 series alloy has the following composition for the elements other than Fe, Mn and Cr, as % by weight:

[0084] Si: 0.80%-1.05%,

[0085] Cu: 0.05%-0.11%,

[0086] Mg 0.30%-0.50%,

[0087] optionally Zn<=0.25%,

[0088] Ti 0.01%-0.10%

[0089] other elements each <=0.05%, total <=0.15%,

[0090] remainder Al

[0091] Preferably, the Si content in this composition is at a maximum 1.05% to improve the looper lines.

[0092] Si and Mg form Mg2Si precipitates that make it possible to obtain the mechanical characteristics after the paints are cured. The excess Si (subtraction of the Mg content from the Si content) improves formability in the T4 state.

[0093] Cu contributes to the mechanical properties in the T4 and T6 states. An excess of Cu may damage corrosion resistance. Preferentially, the Cu is at a maximum 0.15%, more preferentially 0.10%.

[0094] Zn can be added optionally to facilitate recycling but without excess to avoid the phenomenon of corrosion.

[0095] Ti has a role for refining the grains. The maximum Ti content is preferentially 0.10%.

[0096] In an embodiment of each of the alloys described, the Na content is less than or equal to 2.5 ppm, preferably 2.0 ppm. Controlling the maximum Na content contributes to control of waviness. A small Na content can be obtained by reducing the alloy using raw material with very high purity with regard to both aluminum and the addition elements necessary for producing the alloy. A less expensive solution is for the step of producing the aluminum alloy in the 6xxx series to comprise a method for treating the liquid metal making it possible to remove all or part of the Na. A non-limitative example of a method for removing all or part of the Na is that taught by the application WO2022 / 242992. Preferably, the minimum Na is 0.3 ppm, more preferably 0.5 ppm. Such a minimum is a relevant compromise between waviness and the productivity and cost constraints.

[0097] The aluminum alloy is next cast as a plate, preferentially by vertical semicontinuous casting (direct chill casting or DC casting). The preferential dimensions of the plates according to the invention are 200 mm to 600 mm in thickness, 1000 to 3000 mm in width and 2000 to 8000 mm in length. Vertical semicontinuous casting makes it possible to obtain a more homogeneous structure of the sheet than that obtained by continuous casting.

[0098] Advantageously, the sheet manufactured is monolithic. Which is less expensive than a plated sheet.

[0099] The plate is next homogenized at a homogenization temperature, between 540° C. and 580° C., preferentially higher than 550° C., followed by cooling, preferentially 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 beyond the solvus temperature of the alloy, while avoiding local melting and burning. The homogenization temperature is preferentially a maximum of 580° C., preferentially 570° C., and a minimum of 540° C., preferentially a minimum of 550° C. An excessively high or excessively low temperature degrades the mechanical properties of the sheet after aging. Insufficient homogenization temperature or duration may degrade the waviness by not putting sufficiently in solution the Mg2Si, the subsequent precipitates of which will not have the optimum size. The duration of homogenization is preferentially greater than 1 hour. An excessively short duration of homogenization degrades the mechanical properties of the sheet after aging.

[0100] The homogenization of the plate is followed by cooling, preferentially forced, either to a hot-rolling start temperature of 400 to 510° C. or to a temperature below the hot-rolling start temperature. Advantageously, this cooling is direct, i.e. without a second intermediate level stage during homogenization, to either a hot-rolling start temperature of 400 to 510° C. or a temperature below the hot-rolling start temperature in order not to degrade the productivity. Cooling the plate after homogenization makes it possible to obtain optimum-sized precipitates of Mg2Si that then make it possible to control the necessary recrystallization is in the subsequent steps to obtain the required waviness. Hot rolling directly at the homogenization temperature for reasons of productivity results in precipitates of Mg2Si that cause recrystallizations to large grains in the subsequent steps that degrade the waviness. Preferably, the cooling is forced to avoid excessive growth of the Mg2Si precipitates that will be detrimental to the solution heat treatment of the Mg2Si and therefore the mechanical properties. Excessive growth of Mg2Si precipitates degrades the waviness.

[0101] In one embodiment, the homogenized plate is directly cooled to the hot-rolling start temperature in order not to degrade the productivity. This cooling is preferentially forced with a direct cooling speed of at least 150° C. per hour. Advantageously the direct cooling speed is a maximum of 500° C. / h. This cooling can typically be implemented by a machine such as the one described by the application WO2016012691. The minimum speed of 150° C. is a compromise between the precipitation kinetics and productivity. A cooling speed beyond 500° C. / h causes heterogeneities of temperature in the plate that can cause heterogeneity is of precipitations of Mg2Si that may cause a mixed microstructure during subsequent recrystallizations. A mixed microstructure causes inadequate waviness.

[0102] In another embodiment, the homogenized plate is cooled and then reheated to the hot-rolling start temperature. Preferentially, this cooling and then reheating is direct to the hot-rolling start temperature in order not to degrade the productivity. Preferentially, this cooling is forced with fans that propel air at ambient temperature onto the homogenized plate to cool it more quickly than naturally at ambient temperature. This cooling preferentially takes place to a temperature below 300° C. at a preferential speed of 60 to 120° C. / h, more preferentially 70 to 90° C. / h. Preferentially, the plate finishes cooling naturally to ambient temperature. Continuing cooling below the hot-rolling start temperature is advantageous since it makes it possible to continue the Mg2Si precipitation kinetics, which favors recrystallizations during subsequent steps necessary for waviness. The plate is next heated to the hot-rolling start temperature.

[0103] A first hot rolling from the hot-rolling start temperature to a temperature of end of the first hot rolling of 370 to 450° C. is implemented. This first hot rolling is preferentially implemented successively on a rolling mill, two or more reversible hot rolling mills. The end thickness of the first hot rolling is 30 to 50 mm. This first hot rolling is preferentially implemented so that the plate does not heat at each hot-rolling pass. Preferably, the cooling between the hot-rolling start temperature and the temperature of end of first hot rolling 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 temperature of end of first hot rolling is positive or zero. However, a slight reheating of 10° C., i.e. a difference between the hot-rolling start temperature and the temperature of end of first hot rolling at a minimum of −10° C., during the first hot rolling, is acceptable. This hot-rolling method makes it possible to control the continuation of the precipitation of Mg2Si in particular when the plate has been cooled directly to the hot-rolling temperature. This first hot-rolling temperature range helps to control recrystallizations in the subsequent manufacturing steps. Limiting the maximum value of the cooling during the first hot rolling furthermore simplifies the method since this helps not to need intermediate annealing subsequently.

[0104] A second hot rolling it is next implemented from the temperature of end of the first hot rolling to the end of rolling temperature of 250 to 380° C. into a strip under conditions such that the microstructure of the strip is recrystallized or non-recrystallized after the second hot rolling.

[0105] That is to say the microstructure does not comprise recrystallized zones and non-recrystallized zones. A mixed structure after hot rolling will disturb subsequent recrystallizations and degrade the waviness of the sheet in state T4. This second hot rolling is preferentially implemented on a hot rolling mill in tandem including 2, 3, 4, 5, 6 or more hot rolling mills. The strip obtained is next coiled. Preferentially, the coil cools naturally to ambient temperature. Preferentially, no forced cooling or quenching is implemented during the second hot rolling. Preferentially the microstructure is characterized after cooling of the strip to ambient temperature.

[0106] In one embodiment, the temperature of end of hot rolling is above 330° C., preferentially above 340° C., more preferentially above 350° C. A high temperature at end of hot rolling makes it possible to obtain recrystallization to medium grains, which is favorable for final recrystallization to obtain a waviness of less than 0.50 μm. This high temperature simplifies the manufacturing method by not needing intermediate annealing.

[0107] In another embodiment, the temperature of end of hot rolling is below 330° C., preferentially below 325° C., more preferentially below 320° C. A low temperature at end of hot rolling makes it possible to obtain a non-recrystallized microstructure that is favorable for obtaining subsequent recrystallizations to obtain a waviness of less than 0.50 μm.

[0108] The strip is next cold rolled, optionally with an intermediate annealing on a continuous furnace, and without static annealing. An annealing is intermediate when the intermediate annealing is preceded and followed by cold rolling. A static annealing is implemented in a furnace in which the coiled strip undergoes an annealing heat treatment. No static annealing is implemented before, during or after the cold-rolling step. A static annealing does not make it possible to obtain the compromise between surface quality or suitability for bending since the duration of this heat treatment results in large grains.

[0109] Preferentially, the total cold reduction is at least 75%. The total cold reduction is the reduction between the thickness of end of hot rolling and the final thickness. This reduction helps to obtain a waviness of less than or equal to 0.50 μm.

[0110] Preferentially, the annealing on a continuous furnace is a recrystallization annealing. In one embodiment, the annealing on a continuous furnace is done with a PMT (peak metal temperature) below the solvus temperature of the alloy. In another embodiment, the annealing on a continuous furnace also implements a solution heat treatment with a temperature higher than the solvus and lower than the burning temperature. Preferentially, the PMT is selected high to minimize the duration above 350° C. in order to obtain a recrystallization with medium grains.

[0111] A recrystallization on a continuous furnace means that a cold rolling takes place before the solution heat treatment.

[0112] The thickness of the strip after cold rolling is 0.8 to 1.2 mm. An excessively thin thickness cannot be used since the bodywork part does not have sufficient rigidity. An excessively thick thickness makes the bodywork part too heavy for use thereof.

[0113] At least one Intermediate recrystallization takes place in the second hot-rolling and / or cold-rolling steps followed by a reduction by cold rolling of at least 60%. A recrystallization occurred during the second hot-rolling step when the microstructure of the strip after hot rolling is recrystallized. The recrystallization in the cold-rolling step is preferentially obtained with annealing on a continuous furnace. The intermediate recrystallization is followed by a cold rolling, the reduction of at least 60% of which deforms the recrystallized grains in order to obtain a fine-grain recrystallization during the solution heat treatment necessary for waviness. This reduction of at least 60% is the reduction between the intermediate recrystallization thickness and the final thickness.

[0114] The absence of intermediate recrystallization according to the invention causes either insufficient bending or a surface quality characterized by looper lines unsuitable for visible bodywork parts. The intermediate recrystallization according to the invention combined with the cold rolling between the intermediate recrystallization and the solution heat treatment makes it possible to control the final recrystallization that takes place during the solution heat treatment and to obtain the required waviness. Preferentially, the medium grains obtained by this intermediate recrystallization are elongate grains with a length dimension in the long rolling direction of 70 to 200 μm as shown on [FIG. 9]. The ratio of the length to the thickness of the recrystallized grains is from 2 to 5. The length is measured in the long rolling direction. The thickness is measured in the short cross direction. The recrystallized grains obtained after hot rolling have this dimension at half thickness as shown on [FIG. 8]. Excessively long grains leave heredity at the grains resulting from the final recrystallization implemented with the solution heat treatment, this heredity will cause inappropriate waviness. This is the case with the grains obtained by the crystallization with static annealing, as shown on [FIG. 11].

[0115] The reduction of the cold rolling between the intermediate recrystallization and the solution heat treatment is at least 60% in order to sufficiently deform the strip to obtain the final recrystallization during the solution heat treatment necessary to obtain waviness.

[0116] The sheet is next solution heat treated at a solution heat treatment temperature beyond the solvus temperature of the alloy, while avoiding local melting or burning, and then quenched, preferentially in a continuous furnace. The solution heat treatment temperature is preferentially a maximum of 580° C., preferentially 570° C., and a minimum of 540° C., preferentially a minimum of 550° C. An excessively cold solution heat treatment and / or excessively short solution heat treatment degrades the mechanical properties of the sheet by insufficient solution heat treatment. Solution heat treatment that is too hot causes incipient melting degrading the mechanical properties. Solution heat treatment for too long degrades the productivity.

[0117] Solution heat treatment also causes a final recrystallization. Preferentially, it is a recrystallization with fine grains the mean length of which is from 10 to 50 μm, as shown on [FIG. 10]. Preferably, the recrystallized grains have a length to thickness ratio of no more than 2, as shown on [FIG. 10]. The grains are measured in length along the rolling direction and in thickness in the short cross direction. Grains that are longer or more elongate are detrimental to waviness. This microstructure is not modified by the pre-aging and the aging.

[0118] The solution heat treatment temperature is preferentially a minimum of 540° C., preferentially 550° C., and a maximum of 570° C. Preferentially, there is no maintenance at the solution heat treatment temperature to avoid enlarging the grains.

[0119] The quenching is preferentially implemented in air to limit flatness defects.

[0120] Preferably, the sheet is pre-aged. Preferably the sheet is reheated to implement pre-aging at a pre-aging temperature of 50° C. to 120° C., preferentially from 65 to 90° C. for a period of 2 to 16 hours. Heating is useful when the sheet is subjected between quenching and pre-aging to a surface treatment the temperature of which is lower than the pre-aging. Preferentially, pre-aging is obtained by heating without maintenance at the pre-aging temperature and then coiling and cooling to ambient temperature, preferentially for at least 40 hours. Pre-aging stabilizes the natural aging and improves the response of the paints to caring, which is the difference between the yield strength in the T4 state and the yield strength after curing of the paints.

[0121] The sheet ages to the T4 state at ambient temperature between 72 hours and 6 months. This step is a constraint related to storage before forming. The sheet is then in the T4 state.

[0122] The sheet according to the invention can be obtained by the method according to the invention.

[0123] Preferably, the sheet according to the invention is characterized by the mean dimension of the recrystallized grains has a length of 10 to 50 μm, preferably from 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 is less than or equal to 2.

[0124] Preferably, the sheet according to the invention is characterized by a waviness of less than 0.54 μm, preferably 0.50 μm, more preferably 0.48 μm. Reducing the waviness improves the quality of the surface after painting.

[0125] Preferably, the sheet according to the invention obtains a yield strength, in the rolling cross direction, after simulation of the curing of the paints (bake hardening) of least 160 MPa, preferably at least 170 MPa, more preferentially at least 180 MPa, more preferentially at least 190 MPa, more preferentially at least 195 MPa. An excessively low yield strength makes the part sensitive to indentation, i.e. to impact from hailstones. Preferably, the yield strength is below 260 MPa, preferentially 250 MPa, more preferentially 240 MPa, more preferentially 230 MPa. An excessively high yield strength degrades the suitability of the part for bending in the case of accident. The curing of paints is simulated after pre-traction of 2%, in the rolling cross direction, and then heat treatment of 20 minutes at 185° C.

[0126] A bodywork part, preferentially visible, on a vehicle can be produced with the sheet according to the invention. The production method comprises a shaping, preferentially by pressing, and then paint curing. Paint curing, known to a person skilled in the art, corresponds to a heat treatment of 10 to 30 minutes at a temperature of between 17° and 195° C.Embodiment 1

[0127] In a first embodiment, the homogenized plate is directly cooled to the hot-rolling start temperature in accordance with the above description.

[0128] The hot-rolling start temperature is 450 to 520° C., preferentially from 470 to 510° C., more preferentially 470 to 400° C. The temperature of end of hot rolling is below 330° C., preferentially below 325° C., more preferentially below 320° C. An excessively high temperature is liable to cause recrystallization. An excessively low temperature gives rise to excessively great hot-tolling forces. This temperature range makes it possible to obtain a non-recrystallized microstructure of the strip after hot rolling that recrystallizes at medium grains during intermediate annealing to obtain a venous.

[0129] An intermediate annealing on a continuous furnace is implemented during the cold-rolling step. This intermediate annealing is a recrystallization in accordance with the above description. This combination of a non-recrystallized structure after hot rolling with an annealing on a continuous furnace makes it possible to obtain finer grains than compared with a recrystallized structure after hot rolling.Embodiment 2

[0130] In a second embodiment, the homogenized plate is cooled and then reheated to the hot-rolling start temperature in accordance with the above description.

[0131] The hot-rolling start temperature is below 450° C., preferentially 440° C., more preferentially 430° C., more preferentially 420° C. Limiting the hot-rolling start temperature makes it possible to reduce the cooling during the first hot rolling. The temperature of end of hot rolling is above 330° C., preferentially above 340° C., more preferentially above 350° C. The microstructure of the strip is recrystallized with medium grains after the hot rolling. A higher temperature at the end of hot rolling improves the intermediate recrystallization and then the final recrystallization to obtain a waviness of less than 0.50 μm.

[0132] Step g preferentially includes no intermediate annealing on a continuous furnace, which is advantageous for productivity all the more so since it is often expensive equipment sized for the production capacity of the factories that have one available.

[0133] The cold reduction is preferentially greater than 75%, preferentially greater than 80%, in order to improve recrystallization and to obtain better waviness. Cole reduction is a reduction between the end of hot rolling thickness and the final thickness.Embodiment 3

[0134] In a third embodiment, the homogenized plate is directly cooled to the hot rolling start temperature in accordance with the above description.

[0135] The hot-rolling start temperature is below 450° C., preferentially 440° C., more preferentially 430° C., more preferentially 420° C. Limiting the hot-rolling start temperature makes it possible to reduce the cooling during the first hot rolling. The temperature of end of hot rolling is above 330° C., preferentially above 340° C., more preferentially above 350° C. The microstructure of the strip is recrystallized with medium grains after the hot rolling. A higher temperature at the end of hot rolling improves the intermediate recrystallization and then the final recrystallization to obtain a waviness of less than 0.50 μm.

[0136] Step g preferentially includes no intermediate annealing on a continuous furnace, which is advantageous for productivity all the more so since it is often expensive equipment sized for the production capacity of the factories that have one available.

[0137] The cold reduction is preferentially greater than 75%, preferentially greater than 80%, in order to improve recrystallization and to obtain better waviness.Examples

[0138] The disclosure is also illustrated further by the following examples. These examples are only intended to illustrate the invention and not to limit it.

[0139] Plates of various compositions were cast according to the alloys in Table 1. These plates were cast by semicontinuous vertical casting. The examples according to the invention correspond to the plates A, B, C, D, E, F, M, N, O and P.TABLE 1Si (%)Fe (%)Cu (%)Mn (%)Mg (%)Cr (%)Ti (%)Na (ppm)A0.920.210.090.140.420.020.030.3B0.920.210.090.140.410.010.031.0C0.910.290.090.140.430.020.030.4D0.930.190.090.070.410.010.031.4E0.930.190.090.070.410.010.031.0F0.890.260.090.170.410.040.02not measuredG1.300.140.080.070.320.010.031.5H0.940.200.090.080.420.010.030.8I0.920.210.090.140.410.010.031.0J1.070.230.090.140.440.040.030.0K0.900.240.090.170.410.040.03not measuredL0.950.280.040.180.450.030.032.0M0.890.260.090.170.410.040.021.4N0.910.170.090.070.410.010.032.6O0.900.190.090.070.400.010.041.9P0.890.190.090.080.400.010.034.1

[0140] These plates were homogenized at the temperature of 560° C. for 3 hours with the exception of plates D and E, which were homogenized for 12 hours at the temperature of 560° C. Plates D and E were cooled with fans to 300° C. at a cooling rate of 87° C. / h, then these plates were cooled to ambient temperature and then heated to the hot-rolling start temperature. The other plates were called directly to the hot-rolling start temperature with a cooling rate of 150 to 500° C. / h. A first hot rolling is next implemented on a hot reversible rolling mill. The hot-rolling start and end of first hot rolling temperatures are given in table 2. The thickness after this first hot rolling is between 30 and 50 mm. A second hot rolling is next implemented on a tandem rolling mill with 4 rolling mills. The end of hot rolling temperature is given in table 2. The coiled strips were next cooled naturally to ambient temperature. The microstructures were next analyzed. The strips A, B, C, D, E, H, M, N, O and P after hot rolling are recrystallized. The grains are elongate with a length in the long rolling direction of 70 to 200 μm. The ratio of the length to the thickness of the recrystallized grains is from 2 to 5. The strips F, G, I, J, K and L are not recrystallized after hot rolling.TABLE 2Hot-rolling startTemperature ofHot-rolling endtemperatureend of firsttemperatureHot-rolling end(° C.)hot rolling(° C.)thickness [mm]A4094023656B4103903536C4034033746.5D403380 / 4003535E400380 / 4003474F470-490373280-3205G4834023006.2H4083683686.5I4773713097.3J470-490458280-3202.8K470-490380 / 400280-3207.3L470-490380 / 400280-3207.3M4043993596.0N4014053686.0O4004043676.0P4024073676.0

[0141] The strips are next cold rolled in accordance with table 3, which also states the strips that underwent intermediate recrystallization annealing. The microstructures of the strips that underwent intermediate recrystallization annealing were analyzed. Strip F recrystallized after the intermediate annealing. The grains of strip F are elongate with a length in the long rolling direction of 70 to 200 μm. The ratio of the length to the thickness of the recrystallized grains is from 2 to 5. Strips I, K and L recrystallized with grains more than 200 μm.TABLE 3Intermediate-FinalIntermediateannealing aColdFinal coldthicknessannealingthickness (mm)reductionproduction(mm)AN / AN / A84%0.99BN / AN / A84%0.99CN / AN / A82%1.15DN / AN / A77%1.15EN / AN / A71%1.15Fcontinuous2.7480%64%1.00Gcontinuous2.8081%59%1.15HN / AN / A85%1.00Istatic4.0086%75%1.00JN / AN / A71%0.80Kstatic4.584%74%1.15Lstatic4.584%74%1.15MN / AN / A83%1.00NN / AN / A83%1.00ON / AN / A83%1.00PN / AN / A83%1.00

[0142] The strips are next solution heat treated with a temperature of 560° C. (PMT). Strips A, B, C, D, E, F, M, N, O and P recrystallized with fine grains the mean dimension of which is 10 to 50 μm and with a length to thickness ratio of no more than 2.

[0143] The sheets were next pre-aged and then aged for 180 days and are in T4 state. The sheet thus obtained were characterized and the results are in table 4. The bending T.T corresponds to the direction Q-Q in the standards cited. The yield strength was measured with pre-traction of 2%, in the rolling cross direction, and heat treatment of 20 minutes at 185° C. to simulate the effect of Paint caring.TABLE 4Bending T.TRp0.2 (MPa)WavinessLooperstandardized2% + 20′(μm)linesto 1 mm (°)185° C.A0.41 2129206B0.43 2124214C0.44 2132220D0.46>=2134213E0.51>=2134213F0.40>=2Not measured224G0.55 2114214H0.55>=2128228I0.65 2136221J0.44 1Not measured216K0.71>=2Not measured214L0.41>=2114229M0.44Not measuredNot measured228N0.50Not measuredNot measured224O0.47Not measuredNot measured221P0.53Not measuredNot measured224

[0144] The manufacturing method makes it possible to obtain sheets with the required properties.

[0145] FIGS. 13 and 14 show the effect of the waviness observable by the most demanding motor manufacturers. The two figures show the image of a lamp reflected by a pressed sheet, painted with black paint in a manner comparable to what is done in an automobile factory. FIG. 13 is obtained with a sheet that is a counter-example of the invention with waviness greater than 0.54 μm. FIG. 14 is an example of the invention with waviness of less than 0.48 μm. In FIG. 13, the contours of the reflected image of the lamp are made fuzzy by the effect of the waviness, which has a deforming mirror effect. On the other hand, in FIG. 13, the contours of the reflected image of the lamp are much sharper.

Claims

1. A method for manufacturing a metal sheet, comprising:a. Producing a 6xxx series aluminum alloy comprising, as % 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 of the aluminum alloy in a plate, optionally by semicontinuous vertical casting,c. homogenization of the plate at a homogenization temperature, between 540° C. and 580° C., optionally above 550° C., followed by cooling, optionally forced, either to a hot-rolling start temperature of 400 to 520° C. or to a temperature below hot-rolling start temperature,d. First hot rolling from the hot-rolling start temperature to a temperature of end of the first hot rolling of 370 to 450° C.,e. Second hot rolling from the end temperature of first hot rolling to the end of rolling temperature of 250 to 380° C. into a strip under conditions such that the microstructure of the strip is recrystallized or non-recrystallized after the second hot rolling,f. Cold rolling of the strip, with optionally with annealing on a continuous furnace, and without static annealing,g. Solution heat treatment, optionally in air, of the strip into a sheet,h. optionally, pre-aging at a pre-aging temperature of 50 to 120° C. for a period of 2 to 16 hours, optionally obtained by coiling and then cooling to ambient temperature,i. Natural aging from 72 hours to 6 months,wherein at least an intermediate recrystallization occurs in e and / or f followed by a reduction by cold rolling of at least 60%.

2. The Method of claim 1, wherein the aluminum alloy in the 6xxx series furthermore comprises, as % by weight:Mg: 0.25%-1.0%,Si: 0.30%-1.5%,optionally Cu<=0.25%optionally Zn<=0.25%,optionally Ti<=0.15%,optionally V<=0.20%,optionally other elements each <=0.05%, total <=0.15%,remainder Al.

3. The Method of claim 1, wherein the aluminum alloy in the 6xxx series furthermore comprises, as % by weight:Si: 0.30%-1.5%, optionally 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%,remainder Al.

4. The Method of claim 1, wherein the series 6xxx alloy furthermore comprises, as % by weight:Si: 0.5%-1.5%, optionally 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%, optionally 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. The Method of claim 1, wherein the Si content is a maximum of 1.05% as % by weight.

6. The Method of claim 1, wherein the 6xxx series alloy furthermore comprises, as % by weight:Si: 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%,remainder Al.

7. The Method of claim 1, wherein the 6xxx series alloy comprises an Na content <=2.5 ppm by weight, optionally <=2.0 ppm.

8. The Method of claim 1, wherein there is provided a method for treating liquid metal making possible to remove all or some of the Na.

9. The Method of claim 1, wherein the cooling of c is directly to said temperature.

10. The Method of claim 1, wherein the difference between the hot-rolling start temperature and the temperature of end of first hot rolling is a minimum of −10° C. and a maximum of 90° C., optionally 50° C., optionally 40° C., optionally 30° C.

11. The Method of claim 1, wherein total reduction when cold is at least 75%.

12. The Method of claim 1, whereind is optionally a cooling directly from the homogenization temperature to the rolling start temperature,the hot-rolling start temperature is 450 to 520° C.,the temperature of end of hot rolling is below 330° C., optionally below 325° C.,the microstructure of the strip after e is non-recrystallized,g includes an intermediate annealing on a continuous furnace that implements recrystallization of the strip.

13. The of claim 1, whereind is a cooling to a temperature below the hot-rolling start temperature, which is below 450° C.,the temperature of end of second hot rolling is above 340° C., optionally above 350° C.,the microstructure of the strip after e is recrystallized,g optionally includes no intermediate annealing.

14. The Method of claim 1, whereind is a direct cooling from the homogenization temperature to the hot-rolling start temperature, which is below 450° C.,the temperature of end of second hot rolling is above 340° C., optionally above 350° C., the microstructure of the strip after e is recrystallized,g optionally includes no intermediate annealing.

15. A Sheet in T4 state obtained by the method of claim 1.

16. The Sheet of claim 15, wherein a mean dimension of the recrystallized grains, measured with ASTM E112-13, is a length of 10 to 50 μm, optionally 20 μm to 40 μm, and / or the ratio of the length in the long direction to the thickness in the short cross direction of the recrystallized grains is less than or equal to 2.

17. The Sheet of claim 15, wherein the sheet obtains, after pretraction of 2%, in rolling cross direction, then a heat treatment of 20 minutes at 185° C., a yield strength, in the rolling cross direction, of at least 160 MPa and / or no more than 260 MPa.

18. The Sheet of claim 15, wherein waviness, measured with the standard SEP1941 of May 2012, is less than 0.54 μm, optionally less than 0.50 μm, optionally less than 0.48 μm.

19. A Vehicle bodywork part, optionally visible, obtained by a method comprising forming, optionally a pressing, and then a curing of the paint of the metal sheet of claim 15.

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