Manufacturing method for 6XXX alloy sheet with excellent surface quality

Abstract

The present invention discloses a method for producing 6xxx alloy sheet in a T4 temper containing 0.10%-0.40% Fe, 0.05%-0.20% Mn, and 0.01%-0.04% Cr. The method includes homogenization followed by cooling, two-stage hot rolling, and intermediate recrystallization during cold rolling. The sheet according to the present invention offers a compromise between bendability and surface waviness for improved quality after painting.

Description

[Technical Field]

[0001] The present invention relates to the field of aluminum alloy sheets intended for the production, by pressing, of body parts for automobile bodies in white. [Background technology]

[0002] Aluminum alloys are increasingly being used in automotive construction to reduce vehicle weight and therefore fuel consumption and greenhouse gas emissions.

[0003] Aluminum alloys are particularly used to manufacture vehicle body parts, especially visible vehicle body parts, more particularly exterior parts. Visible vehicle body parts, more particularly exterior parts, must comply with technical specifications so that, after painting, these parts have a required appearance that is satisfactory to the vehicle driver. Conventionally, surface quality is characterized by measuring roping, but this characterization does not reveal all surface defects.

[0004] EP 1967598 A1 discloses a 6000-type aluminum alloy sheet containing Si and Mg as major alloying elements, having excellent formability sufficient to allow bending when flat, excellent dent resistance, and excellent adaptability to hardening during paint baking. This application discloses a method for producing an aluminum alloy sheet, which comprises subjecting an ingot to a homogenizing treatment, cooling at a cooling rate of 100°C / hour or more to a temperature below 350°C, optionally to ambient temperature, heating again to a temperature of 300-500°C and subjecting the hot-rolled product 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.

[0005] EP 3485055 A1 discloses a method for producing a 6xxx series aluminum sheet, comprising the steps of homogenizing a 6xxx series aluminum ingot, directly cooling the homogenized ingot to the hot rolling start temperature at a cooling rate of 150°C / h to 2000°C / h, hot rolling the ingot to the final hot rolling thickness and coiling under conditions such that at least 50% crystallization is obtained at the final hot rolling thickness, and cold rolling to obtain a cold-rolled sheet. The method according to the present invention is particularly useful for producing sheet for the automotive industry, which combines high tensile strength, good forming properties suitable for cold pressing operations, as well as high surface quality and high corrosion resistance, with high productivity.

[0006] WO 2018 / 206696 discloses a method for producing a rolled sheet of an aluminum alloy having excellent formability and good compatibility with paint bake hardening, the method comprising the steps of: (a) casting an ingot of an Al-Si-Mg aluminum alloy containing, by weight, 1.0% to 1.50% Si and 0.10% to 0.40% Mg; (b) heating the ingot to a temperature greater than 550°C, maintaining the ingot at a temperature greater than 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 6 hours. (c) hot rolling the ingot in one or more rolling passes to an intermediate size of 15 mm to 40 mm, wherein the hot rolling mill has an exit temperature of 370°C to 480°C; (d) hot rolling again from the intermediate gauge to a final hot rolled gauge in one or more rolling passes, wherein the hot rolling mill has an exit temperature of 310°C to 400°C; (e) cooling the hot rolled material from the hot rolling mill exit temperature to ambient temperature to the final hot rolled gauge; and (f) cold rolling the hot rolled product to a final gauge cold rolled product.

[0007] US Patent Application Publication No. 2021 / 0340654 discloses a method for producing a 6xxx series aluminum sheet, which contains, in weight percent, Si: 0.4-0.7, Mg: 0.2-0.4, Mn: 0.05-0.30, Fe: 0.03-0.4, Cu: max. 0.3, Cr: max. 0.05, Zn: max. 0.15, Ti: max. 0.1 weight percent, with the remainder being aluminum and impurities each up to 0.05, totaling 0.15. The method includes the steps of homogenizing an ingot made of a 6xxx series aluminum alloy containing unavoidable impurities, rough hot rolling the ingot in a reversing rolling mill to a rough hot rolling exit thickness at a rough hot rolling exit temperature of less than 420°C, finish hot rolling the ingot in a tandem rolling mill to a final hot rolling thickness, coiling the ingot to the final hot rolling thickness at a hot rolling exit temperature of less than 300°C, and cold rolling to obtain a cold-rolled sheet. The product obtained according to the method of the present invention is particularly useful for automobile hood interiors because it has the mechanical properties necessary for pedestrian safety and surface quality. This application aims to provide a sheet for interior parts, such as hood linings, that are visible only when the vehicle hood is open. Such parts do not have to compromise on pedestrian safety, for example, and therefore do not have the same level of surface quality requirements as exterior parts.

[0008] A new requirement emerges: the waviness of the sheet surface in temper T4. If the waviness of the sheet in temper T4 is too great, a distorted mirror effect appears on the painted bodywork parts, which can be unpleasant for the most demanding car drivers. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] European Patent Application Publication No. 1967598 [Patent Document 2] European Patent Application Publication No. 3485055 [Patent Document 3] International Publication No. 2018 / 206696 [Patent Document 4] U.S. Patent Application Publication No. 2021 / 0340654 Summary of the Invention [Problem to be solved by the invention]

[0010] The problem to be solved is to develop a 6xxx series alloy sheet that aims to be an outstanding compromise between: Waviness of sheet of T4 temper less than 0.54 μm, Bending angle of the sheet in temper T4 after 6 months of natural ageing of 125° or more. [Means for solving the problem]

[0011] [Disclosure of the Invention] One subject of the present invention is a method for producing ... pharmaceutical composition comprising the following successive steps: a. by weight percent i. 0.10%~0.40% Fe ii. 0.05%~0.20% Mn iii. 0.01%~0.04% Cr A process for producing 6xxx series aluminum alloys, including b. Casting of the aluminum alloy in plates, preferably by semi-continuous vertical casting; c. homogenization of the plate at a homogenization temperature of 540°C to 580°C, preferably above 550°C, followed by a preferably forced cooling step to a hot rolling start temperature of 400 to 510°C or to a temperature below the hot rolling start temperature; d. A first hot rolling step from a hot rolling start temperature to a first hot rolling finish temperature of 370 to 450°C; e. A second hot rolling step of the strip from the first hot rolling finish temperature to a rolling finish temperature of 250-380°C under conditions such that the microstructure of the strip is either recrystallized or not recrystallized after the second hot rolling; f. cold rolling of strip, optionally with annealing in a continuous furnace and without static annealing; g. Solution treatment followed by quenching of the strip into a sheet, preferably in air; h. a pre-aging step, preferably at a pre-aging temperature of 50-120°C for a period of 2-16 hours, suitably obtained by coiling followed by cooling to ambient temperature; i. Natural aging process of 72 hours to 6 months; A method for producing a rolled sheet made of a 6xxx series aluminum alloy, comprising: A method of manufacturing wherein at least an intermediate recrystallization occurs in steps e and / or f, followed by a cold rolling reduction of at least 60%.

[0012] Another subject of the invention is a sheet obtainable by the method according to the invention.

[0013] Another subject of the invention is a vehicle body part, preferably visible, obtained by a method comprising shaping, preferably pressing, followed by paint baking on a sheet according to the invention. [Brief explanation of the drawings]

[0014] [Figure 1] FIG. 1 shows the microstructure of a strip of temper T4. [Figure 2] FIG. 1 shows the microstructure of deformed grains. [Figure 3] FIG. 1 shows the microstructure of recovered particles. [Figure 4] FIG. 1 shows the microstructure of elongated recrystallized grains. [Figure 5] FIG. 1 shows a microstructure of large equiaxed recrystallized grains. [Figure 6] FIG. 1 shows a microstructure of small equiaxed recrystallized grains. [Figure 7] FIG. 1 shows a partially recrystallized or mixed microstructure. [Figure 8] FIG. 1 shows the microstructure of the recrystallized strip after hot rolling. [Figure 9] FIG. 1 shows the microstructure of the recrystallized strip after annealing in a continuous furnace. [Figure 10] FIG. 1 shows the microstructure of the recrystallized strip after static annealing. [Figure 11] FIG. 1 shows examples of samples subjected to looper line characterization, in classes 1, 2 and 3 (1: average to 3: excellent). [Figure 12] FIG. 10 shows the effect of Na on waviness. [Figure 13] FIG. 1 shows a counterexample to the present invention, a pressed and painted sheet. [Figure 14] FIG. 1 shows a pressed and painted sheet, an example of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0015] Detailed Description of the Invention All aluminum alloys discussed below are designated according to the rules and nomenclature set forth in the Register Record Series, published periodically by the Aluminum Association, unless otherwise specified. Compositions are expressed in weight percent unless otherwise specified. The expression 1.4Cu means that the copper content, expressed in weight percent, is 1.4%. For Na, compositions are expressed in ppm by weight. The expression 1.4Na means that the sodium content, expressed in ppm by weight, is 1.4 ppm.

[0016] Groups of alloys, also called systems, are defined in EN 573-1 (2005).

[0017] The metallurgical conditions discussed are expressed according to European Standard EN 515 (2017).

[0018] The static tensile mechanical properties, i.e., tensile strength Rm, conventional elastic limit at 0.2% elongation Rp0.2, plastic elongation Ag%, and elongation at break A%, were measured by tensile tests according to NF EN ISO6892-1(2018), with the sampling and direction of the test defined in EN485-1(2009).

[0019] The bending angle, called the α norm, is measured by a three-point bending test according to the procedures of NF EN ISO 7438 (2005) and VDA 238-100 version 2010 and VDA 239-200 version 2017.

[0020] Particle size is measured in accordance with ASTM E112-13(2021).

[0021] Unless otherwise specified, the definitions in EN12258-1(2012) apply.

[0022] Looper lines are measured in the following way: Strips measuring approximately 270 mm (transverse direction) and 50 mm (rolling direction) are cut into thin sheets. A 15% tensile pre-deformation is then applied in the direction perpendicular to the rolling direction, i.e., along the length of the strip. The strip is then subjected to the action of P800 type abrasive paper to reveal the looper lines. The looper lines are then visually assessed and classified on a scale from 1 (noticeable looper lines) to 3 (absence of looper lines). Examples of looper lines corresponding to values ​​1 to 3 are shown in Figure 11.

[0023] The surface waviness Wsa(1-5) is measured using standard SEP1941 of May 2012. The measurements of waviness Wsa(1-5) are carried out on sheets of temper T4 after a deformation of 15% in the transverse rolling direction. The waviness is the average of 14 measurements over a length of 30 mm, each measurement length being at least 2.5 mm away from the other measurements.

[0024] Aluminum and aluminum alloys are polycrystalline materials whose properties and arrangement can be modified by deforming the metal (e.g., rolling, extruding, forging) or by applying heat (e.g., annealing). During deformation of aluminum alloys, the free energy of the crystalline material can increase, for example, through crystal slip. Crystal slip involves the movement of dislocations in several planes and directions within each crystal. When crystal slip occurs during plastic deformation, the dislocation density and crystal rotation within the material increase. Crystal rotation with deformation is one of the reasons why polycrystalline materials develop texture, i.e., a non-random orientation of the crystals, also called grains. Dislocations are therefore defects in the crystals of the grains.

[0025] The microstructure of polycrystalline materials, such as aluminum alloys, changes depending on their thermomechanical history. For example, aluminum alloys have a deformed microstructure after deformation, a recovered microstructure after recovery annealing, and a recrystallized microstructure after recrystallization annealing, which are described in more detail below. An example of a microstructure containing deformed grains is shown in Figure 2. In the example shown, the microstructure 2 includes multiple deformed grains 12, each with a grain boundary 10. Due to deformation, the interior regions of the deformed grains 12 have a high dislocation density, indicated by shading 14 in Figure 2.

[0026] To reduce the free energy of a deformed material, it can be annealed. Annealing involves heating the deformed material to a high temperature. Two types of annealing are commonly used to process aluminum alloys: recovery annealing and recrystallization annealing. In recovery annealing, the aluminum alloy is heated to a temperature where the grain boundaries of the deformed grains are generally maintained, but dislocations within the deformed grains move toward lower-energy configurations. These lower-energy configurations within the grains are called subgrains or cells. Therefore, the grains created by recovery annealing are commonly referred to as recovered grains. An example of a microstructure containing recovered grains is shown in Figure 3. In the example shown, the recovered microstructure 3 contains recovered grains 22. The recovered grains 22 generally share the same grain boundaries 10 as the deformed grains 12, but subgrains 16 have formed within the recovered grains 12 due to the recovery annealing.

[0027] During recrystallization annealing, the aluminum alloy is heated to a temperature at which new grains are formed from the deformed grains 12 and / or recovered grains 22. These new grains are called recrystallized grains. Recrystallization annealing results in the formation of a material with recrystallized grains. Examples of microstructures containing recrystallized grains are shown in Figures 4, 5, and 6. In the examples shown, microstructure 4 contains elongated recrystallized grains 32c (Figure 4), microstructure 5 contains large equiaxed recrystallized grains 32d (Figure 5), and microstructure 6 contains small equiaxed recrystallized grains 32e (Figure 6). A microstructure is recrystallized if at least 90% of the observed surface is recrystallized. A microstructure is non-recrystallized if 10% or less of the observed surface is recrystallized.

[0028] In some circumstances, annealing can produce a partially recrystallized or mixed material, an example of which is shown in Figure 7. In the example shown, the partially recrystallized or mixed microstructure 7 contains a mixture of recovered grains 22 and recrystallized grains 32.

[0029] The ambient temperature is any temperature between 5 and 35°C that is suitable for human operation.

[0030] [method] The present invention is based on the observation made by the applicant that, thanks to a suitable composition and manufacturing method, it is absolutely possible to manufacture sheets that, after painting, have an excellent surface quality while maintaining excellent bending suitability, the method being preferably specialized for sheets intended in particular for external, visible body parts.

[0031] The method of manufacturing sheet according to the present invention comprises producing a 6xxx series aluminium alloy comprising, in weight %: 0.10%~0.40% Fe 0.05%~0.20% Mn 0.01%~0.04% Cr.

[0032] Mn and Cr are precipitated during the manufacturing process. They should preferably precipitate in the form of dispersoids, small precipitates with a typical average size of 0.1-0.3 μm relative to the grain size. These dispersoids contribute to the control of various recrystallizations that occur during the manufacturing process and affect the waviness of the sheet. Fe also precipitates and contributes to the control of various recrystallizations. The content of these elements is a compromise that must be compatible with the manufacturing process.

[0033] If the Mn and Cr contents are excessive, recrystallization during the manufacturing process will not occur properly, resulting in worse waviness. The maximum Mn content is 0.20%, preferably 0.19%, preferably 0.18%, more preferably 0.17%, more preferably 0.16%, and more preferably 0.15%. The maximum Cr content is 0.04%, preferably 0.03%, and more preferably 0.02%. If the contents of these elements are insufficient, the alloy will tend to crystallize in the form of large grains during solution treatment, which will also worsen waviness. The minimum Mn content is 0.05%. The minimum Cr content is 0.01%.

[0034] An excessively low Fe ratio makes the alloy particularly expensive. The addition of Fe, which forms insoluble precipitates, also helps control recrystallization, but excess Fe reduces the bendability of the sheet in the T4 temper. A high maximum Fe content advantageously makes the alloy more recyclable during production of the alloy according to the invention. The minimum Fe content is 0.10%, preferably 0.15%, and 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%, and more preferably 0.30%.

[0035] According to EN573-1 (2005), within the 2xxx to 8xxx groups, the alloy designation is determined by the additive element present in the highest average percentage (Mg2Si for 6xxx alloys). If the highest average percentage is common to multiple additive elements, the group is selected in the following order: Cu, Mn, Si, Mg, Mg2Si, Zn, etc. Preferably, 6xxx series alloys contain 0.25% to 1.0% Mg and 0.30% to 1.5% Si. More preferably, 6xxx series aluminum alloys further contain Cu≦0.25%, Zn≦0.25%, Ti≦0.15%, V≦0.20%, and other elements each≦0.05%, total≦0.15%, with the remainder being Al.

[0036] Preferably, Si is at most 1.05% to improve the surface quality characterized by looper lines.

[0037] In one embodiment, the 6xxx series aluminum alloy further comprises, in wt. %: Si: 0.30% to 1.5%, preferably 0.30% to 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%, total ≦0.15% The rest is Al.

[0038] In one embodiment, the 6xxx series aluminum alloy further comprises, in wt. %: Si: 0.5% to 1.5%, preferably 0.5% to 1.05%, Cu≦0.20%, Mg: 0.25% to 0.6%, Zn≦0.20%, Ti≦0.15%, other elements each≦0.05%, total≦0.15%, balance 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%, balance Al, Alternatively, Si: 0.8% to 1.5%, preferably 0.8% to 1.05%, Cu: 0.01% to 0.11%, Mg: 0.45% to 0.7%, Zn≦0.25%, Ti≦0.10%, other elements each ≦0.05%, total ≦0.15%, the balance being 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%, balance 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%, balance Al.

[0039] Preferably, the Si content is at most 1.05% to improve the looper line.

[0040] Preferably, the 6000 series alloy has the following composition, in weight percent, for elements other than Fe, Mn, and Cr: Si: 0.80%~1.05%, Cu: 0.05%~0.11%, Mg: 0.30%~0.50%, Optionally, Zn≦0.25%; Ti: 0.01%~0.10%, Other elements each ≦0.05%, total ≦0.15% The rest is Al.

[0041] Preferably, the Si content in this composition is at most 1.05% to improve the looper line.

[0042] Si and Mg form Mg2Si precipitates that allow the steel to achieve the desired mechanical properties after paint baking. Excess Si (Si content minus Mg content) improves formability in T4 temper.

[0043] Cu contributes to mechanical properties in T4 and T6 tempers. Excessive Cu can impair corrosion resistance. Preferably, Cu is at most 0.15%, more preferably 0.10%.

[0044] Zn can be optionally added to facilitate recycling, but cannot be added in excess to avoid corrosion phenomena.

[0045] Ti plays a role in grain refinement, and the maximum content of Ti is preferably 0.10%.

[0046] In each of the described alloy embodiments, the sodium content is 2.5 ppm or less, preferably 2.0 ppm. Controlling the maximum sodium content contributes to waviness control. A low sodium content can be achieved by reducing the alloy using very high purity raw materials, both with respect to the aluminum and the additional elements required for the alloy's production. A cheaper solution is to include a liquid metal processing method in the production of 6xxx series aluminum alloys that allows for the removal of all or part of the sodium. A non-limiting example of a method for removing all or part of the sodium is the method taught in WO 2022 / 242992. Preferably, the minimum sodium value is 0.3 ppm, more preferably 0.5 ppm. Such a minimum value is a good compromise between waviness, productivity, and cost constraints.

[0047] 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 plate according to the present invention are a thickness of 200 mm to 600 mm, a width of 1000 to 3000 mm, and a length of 2000 to 8000 mm. Vertical semi-continuous casting makes it possible to obtain a sheet with a more uniform structure than that obtained by continuous casting.

[0048] Advantageously, the sheets produced are monolithic, which is cheaper than plated sheets.

[0049] The plate is then homogenized at a homogenization temperature of 540°C to 580°C, preferably higher than 550°C, followed by cooling, preferably forced cooling, to a temperature below the hot rolling start temperature of 400°C to 520°C. The homogenization temperature exceeds the solvus temperature of the alloy while avoiding localized melting and combustion. The homogenization temperature is preferably a maximum of 580°C, preferably 570°C, and a minimum of 540°C, preferably 550°C. Excessively high or low temperatures can degrade the mechanical properties of the aged sheet. Insufficient homogenization temperature or duration can worsen waviness due to insufficient dissolution of Mg2Si, and subsequent precipitates will not have optimal size. The homogenization duration is preferably greater than one hour. An excessively short homogenization duration can degrade the mechanical properties of the aged sheet.

[0050] Following homogenization, the plate is cooled, preferably forcedly, to the hot rolling start temperature of 400-510°C or below. Advantageously, to avoid a decrease in productivity, this cooling is performed directly, i.e., without a second intermediate stage during homogenization, to the hot rolling start temperature of 400-510°C or below. Cooling the plate after homogenization allows for optimally sized Mg2Si precipitates, which allows for the necessary recrystallization control and desired waviness in subsequent processes. Direct hot rolling at the homogenization temperature due to productivity concerns can result in Mg2Si precipitates that recrystallize into large grains in subsequent processes, worsening waviness. Preferably, forced cooling is used to avoid excessive growth of Mg2Si precipitates, which is detrimental to the Mg2Si solution treatment and, therefore, to the mechanical properties. Excessive growth of Mg2Si precipitates aggravates waviness.

[0051] In one embodiment, to avoid reducing productivity, the homogenized plate is directly cooled to the hot rolling start temperature. This cooling is preferably performed by forced cooling at a direct cooling rate of at least 150°C per hour. Advantageously, the direct cooling rate is up to 500°C / hour. This cooling can typically be performed using equipment such as that described in WO 2016 / 012691. The minimum rate of 150°C represents a compromise between precipitation behavior and productivity. Cooling rates exceeding 500°C / hour can cause temperature inhomogeneities within the plate, leading to inhomogeneous Mg2Si precipitates and potentially resulting in a mixed microstructure during subsequent recrystallization. The mixed microstructure can lead to undesirable waviness.

[0052] In another embodiment, the homogenized plate is cooled and then reheated to the hot rolling start temperature. Preferably, this cooling and subsequent reheating is performed directly to the hot rolling start temperature to avoid reducing productivity. Preferably, this cooling is performed by a fan blowing ambient temperature air onto the homogenized plate, resulting in a cooling rate faster than natural cooling at ambient temperature. This cooling is preferably performed at a rate of 60-120°C / hr, more preferably 70-90°C / hr, to a temperature below 300°C. Preferably, the plate is allowed to complete its natural cooling to ambient temperature. Continuing the cooling below the hot rolling start temperature is advantageous because it allows the MgSi precipitation behavior to continue, favoring recrystallization during the subsequent process required for waviness. The plate is then heated to the hot rolling start temperature.

[0053] A first hot rolling is performed from the hot rolling start temperature to a first hot rolling end temperature of 370-450°C. This first hot rolling is preferably performed continuously using one rolling mill or two or more reversing hot rolling mills. The final thickness of the first hot rolling is 30-50 mm. This first hot rolling is preferably performed so that the plate is not heated 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 during the first hot rolling is permitted, i.e., a minimum difference of -10°C between the hot rolling start temperature and the first hot rolling end temperature. This hot rolling method allows for the continued control of Mg2Si precipitation, especially if the plate is cooled directly to the hot rolling temperature. This first hot rolling temperature range helps control recrystallization in subsequent manufacturing steps. Limiting the maximum cooling during the first hot rolling further simplifies the method, as it helps eliminate the need for subsequent intermediate annealing.

[0054] The strip is then subjected to a second hot rolling from the first hot rolling finish temperature to a rolling finish temperature of 250-380°C under conditions such that the microstructure of the strip is either recrystallized or non-recrystallized after the second hot rolling. That is, the microstructure does not contain recrystallized or non-recrystallized regions. A mixed structure after hot rolling prevents subsequent recrystallization and exacerbates waviness in the sheet in the T4 temper. This second hot rolling is preferably performed in a tandem hot rolling mill, including two, three, four, five, six, or more hot rolling mills. The resulting strip is then coiled. Preferably, the coil is naturally cooled to ambient temperature. Preferably, no forced cooling or quenching is performed during the second hot rolling. Preferably, the microstructure is characterized after the strip has cooled to ambient temperature.

[0055] In one embodiment, the hot rolling finish temperature is greater than 330° C., preferably greater than 340° C., and more preferably greater than 350° C. A high hot rolling finish temperature allows for recrystallization to a medium grain, which is preferable for final recrystallization to obtain a waviness of less than 0.50 μm. This high temperature can simplify the manufacturing process by eliminating the need for intermediate annealing.

[0056] In another embodiment, the hot rolling finish temperature is less than 330° C., preferably less than 325° C., more preferably less than 320° C. A low hot rolling finish temperature allows for a non-recrystallized microstructure, which is favorable for obtaining a waviness of less than 0.50 μm with subsequent recrystallization.

[0057] The strip is then cold rolled, optionally with intermediate annealing in a continuous furnace, without static annealing. Annealing is intermediate if cold rolling is performed before or after the intermediate annealing. Static annealing is performed in a furnace where the coiled strip undergoes an annealing heat treatment. No static annealing is performed before, during or after the cold rolling process. Static annealing does not allow a compromise to be obtained between surface quality and bendability, since the duration of this heat treatment leads to large grains.

[0058] Preferably, the total cold reduction is at least 75%, which is the reduction between the hot rolled thickness and the final thickness, and is useful for achieving a waviness of 0.50 μm or less.

[0059] Preferably, the continuous furnace annealing is a recrystallization annealing. In one embodiment, the continuous furnace annealing is performed at a PMT (peak metal temperature) below the solvus temperature of the alloy. In another embodiment, the continuous furnace annealing also includes a solution treatment above the solvus temperature but below the melting temperature. Preferably, the PMT is selected high to minimize the time above 350°C in order to obtain medium-grain recrystallization. Continuous furnace recrystallization means that cold rolling is performed before the solution treatment.

[0060] The thickness of the strip after cold rolling is 0.8 to 1.2 mm. If the thickness is too thin, the body part will not be rigid enough to be used. If the thickness is too thick, the body part will be too heavy to be used.

[0061] In the second hot and / or cold rolling step, at least one intermediate recrystallization is performed, followed by cold rolling to a reduction of at least 60%. If the microstructure of the hot-rolled strip is recrystallized, recrystallization occurs during the second hot rolling step. The recrystallization in the cold rolling step is preferably achieved by annealing in a continuous furnace. The intermediate recrystallization is followed by cold rolling, with a reduction of at least 60% to deform the recrystallized grains in order to obtain fine grain recrystallization during the solution treatment required for waviness. This reduction of at least 60% is the reduction between the intermediate recrystallization thickness and the final thickness.

[0062] The lack of intermediate recrystallization according to the present invention leads to insufficient bending or a surface quality characterized by visible looper lines that are unsuitable for vehicle body parts. The intermediate recrystallization according to the present invention, combined with cold rolling between the intermediate recrystallization and the solution treatment, allows for control of the final recrystallization that occurs during the solution treatment and allows for the desired waviness. Preferably, the intermediate grains obtained by this intermediate recrystallization are elongated grains with a length dimension of 70 to 200 μm in the longitudinal rolling direction, as shown in [Figure 9]. The recrystallized grains have a length-to-thickness ratio of 2 to 5. The length is measured along the longitudinal rolling direction, and the thickness is measured along the transverse direction. The recrystallized grains obtained after hot rolling have this dimension at half the thickness, as shown in [Figure 8]. Excessively elongated grains retain characteristics of the grains resulting from the final recrystallization performed during the solution treatment, which can cause inappropriate waviness. This is the case for grains obtained by crystallization during static annealing, as shown in [Figure 11].

[0063] The cold rolling reduction between the intermediate recrystallization and the solution treatment is at least 60% in order to sufficiently deform the strip during the solution treatment to obtain the waviness required to obtain the final recrystallization.

[0064] The sheet is then solution-treated at a solution-treatment temperature above the solvus temperature of the alloy, while avoiding localized melting or burning, and then quenched, preferably in a continuous furnace. The solution-treatment temperature is preferably a maximum of 580°C, preferably a maximum of 570°C, and a minimum of 540°C, preferably a minimum of 550°C. An excessively low solution-treatment temperature and / or an excessively short solution-treatment temperature will reduce the mechanical properties of the sheet due to insufficient solution treatment. A solution-treatment temperature that is too high will cause melting, which will reduce the mechanical properties. A solution-treatment that is too long will reduce productivity.

[0065] Solution treatment also induces final recrystallization, preferably with fine grains having an average length of 10 to 50 μm, as shown in FIG. 10. As shown in FIG. 10, the recrystallized grains preferably have a length-to-thickness ratio of at most 2. The grains are measured by their length along the rolling direction and their thickness in the transverse direction. Longer or more elongated grains adversely affect waviness. This microstructure is not altered by pre-aging or aging.

[0066] The solution treatment temperature is preferably a minimum of 540° C., preferably 550° C., and a maximum of 570° C. Preferably, the solution treatment temperature is not maintained to prevent grain growth.

[0067] Quenching is preferably carried out in air to reduce flatness defects.

[0068] Preferably, the sheet is pre-aged. Preferably, the sheet is heated to a pre-aging temperature of 50°C to 120°C, preferably 65°C to 90°C, for a period of 2 to 16 hours to perform the pre-aging. Heating is useful if the sheet is subjected to a surface treatment at a temperature lower than that of the pre-aging between quenching and pre-aging. Preferably, pre-aging is achieved by heating without maintaining the sheet at the pre-aging temperature, followed by coiling and cooling to ambient temperature, preferably for at least 40 hours. Pre-aging stabilizes natural aging and improves the response to paint baking, i.e., the difference between the elastic limit in temper T4 and the elastic limit after paint baking.

[0069] The sheet is aged at ambient temperature for 72 hours to 6 months to a T4 temper. This step is subject to storage constraints before molding. The sheet then reaches a T4 temper.

[0070] The sheet according to the invention can be obtained by the method according to the invention.

[0071] Preferably, the sheet according to the present invention is characterized in that the recrystallized grains have an average length of 10 to 50 μm, preferably 20 to 40 μm, and / or the ratio of the longitudinal length of the recrystallized grains to the transverse thickness of the recrystallized grains is 2 or less.

[0072] 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 surface quality after painting.

[0073] Preferably, the sheet according to the invention has an elastic limit in the transverse direction of rolling of at least 160 MPa, preferably at least 170 MPa, more preferably at least 180 MPa, more preferably at least 190 MPa, and even more preferably at least 195 MPa after simulated paint baking (bake hardening). An excessively low elastic limit makes the part sensitive to dents, i.e., hail impacts. Preferably, the elastic limit is less than 260 MPa, preferably less than 250 MPa, more preferably less than 240 MPa, and even more preferably less than 230 MPa. An excessively high elastic limit reduces the part's ability to bend in the event of an accident. Paint baking is simulated in the transverse direction after 2% pretraction and subsequent heat treatment at 185°C for 20 minutes.

[0074] The sheet according to the invention can be used to manufacture preferably visible body parts of a vehicle. The manufacturing method comprises forming, preferably by pressing, followed by paint baking. Paint baking, as known to those skilled in the art, corresponds to a heat treatment at a temperature of 170-195°C for 10-30 minutes.

[0075] [Embodiment 1] In a first embodiment, the homogenized plate is cooled directly to the hot rolling start temperature as described above.

[0076] The hot rolling start temperature is 450 to 520°C, preferably 470 to 510°C, and more preferably 470 to 400°C. The hot rolling finish temperature is less than 330°C, preferably less than 325°C, and more preferably less than 320°C. An excessively high temperature is likely to cause recrystallization. An excessively low temperature induces excessively large hot rolling forces. This temperature range allows for the strip to have a non-recrystallized microstructure after hot rolling, which recrystallizes with medium grains during intermediate annealing to obtain waviness.

[0077] An intermediate annealing in a continuous furnace is carried out during the cold rolling process, this intermediate annealing being a recrystallization according to the above description. This combination of the non-recrystallized structure after hot rolling and the annealing in a continuous furnace makes it possible to obtain finer grains compared to the recrystallized structure after hot rolling.

[0078] [Embodiment 2] In a second embodiment, the homogenized plate is cooled and then reheated to the hot rolling start temperature as described above.

[0079] The hot rolling start temperature is less than 450°C, preferably 440°C, more preferably 430°C, and even more preferably 420°C. Limiting the hot rolling start temperature allows for reduced cooling during the first hot rolling. The hot rolling finish temperature is greater than 330°C, preferably greater than 340°C, and even more preferably greater than 350°C. The microstructure of the strip is medium-grained and recrystallized after hot rolling. A higher temperature at the end of hot rolling improves the intermediate recrystallization and then the final recrystallization, resulting in a waviness of less than 0.50 μm.

[0080] Step g preferably does not include any intermediate annealing in a continuous furnace, which is all the more advantageous in terms of productivity since the production capacity of factories with continuous furnaces is often an expensive, moderately sized facility.

[0081] The cold reduction is preferably greater than 75%, preferably greater than 80%, to improve recrystallization and obtain better waviness. The cold reduction is the reduction between the thickness at the end of hot rolling and the final thickness.

[0082] [Embodiment 3] In a third embodiment, the homogenized plate is cooled directly to the hot rolling start temperature as described above.

[0083] The hot rolling start temperature is less than 450°C, preferably 440°C, more preferably 430°C, and even more preferably 420°C. Limiting the hot rolling start temperature allows for reduced cooling during the first hot rolling. The hot rolling finish temperature is greater than 330°C, preferably greater than 340°C, and even more preferably greater than 350°C. The microstructure of the strip is medium-grained and recrystallized after hot rolling. A higher temperature at the end of hot rolling improves the intermediate recrystallization and then the final recrystallization, resulting in a waviness of less than 0.50 μm.

[0084] Step g preferably does not include any intermediate annealing in a continuous furnace, which is all the more advantageous in terms of productivity since the production capacity of factories with continuous furnaces is often an expensive, moderately sized facility.

[0085] The cold rolling reduction is preferably greater than 75%, preferably greater than 80%, to improve recrystallization and obtain better waviness. [Example]

[0086] The present disclosure is further illustrated by the following examples, which are intended to illustrate the present invention only and are not intended to limit the present invention.

[0087] Plates of various compositions were cast according to the alloys in Table 1. These plates were cast by semi-continuous vertical casting. Examples according to the present invention correspond to plates A, B, C, D, E, F, M, N, O and P.

[0088] [Table 1]

[0089] The plates were homogenized at 560°C for 3 hours, except for plates D and E, which were homogenized at 560°C for 12 hours. Plates D and E were cooled to 300°C using a fan at a cooling rate of 87°C / h, and then the plates were cooled to ambient temperature and then heated to the hot rolling start temperature. The other plates were cooled directly to the hot rolling start temperature at cooling rates of 150-500°C / h.

[0090] Next, the first hot rolling was performed in a hot reversing mill. The starting and finishing temperatures for the first hot rolling are listed in Table 2. The thickness after this first hot rolling was 30-50 mm. Next, the second hot rolling was performed in a tandem mill with four rolling mills. The finishing temperatures are listed in Table 2. The coiled strip was then naturally cooled to ambient temperature. The microstructure was then analyzed. After hot rolling, strips A, B, C, D, E, H, M, N, O, and P were recrystallized. The grains were elongated to lengths of 70-200 μm along the rolling direction. The length-to-thickness ratio of the recrystallized grains was 2-5. Strips F, G, I, J, K, and L were not recrystallized after hot rolling.

[0091] [Table 2]

[0092] The strips are then cold rolled according to Table 3, which also specifies the strips that have undergone intermediate recrystallization annealing. The microstructures of the strips that have undergone intermediate recrystallization annealing are analyzed. Strip F is recrystallized after the intermediate annealing. The grains of Strip F are elongated to lengths of 70 to 200 μm in the rolling direction. The length-to-thickness ratio of the recrystallized grains is 2 to 5. Strips I, K, and L are recrystallized with grains larger than 200 μm.

[0093] [Table 3]

[0094] The strips are then solution-treated at a temperature (PMT) of 560 °C. Strips A, B, C, D, E, F, M, N, O and P are recrystallized with fine grains having an average size of 10-50 μm and a length to thickness ratio of at most 2.

[0095] The sheet is then pre-aged and then aged for 180 days to obtain temper T4. The sheet thus obtained is characterized and the results are shown in Table 4. The bending TT corresponds to the direction QQ of the quoted standard. The elastic limit was measured using a pretraction of 2% in the transverse direction of rolling and a heat treatment of 20 minutes at 185 °C to simulate the effect of paint baking.

[0096] [Table 4]

[0097] The manufacturing method makes it possible to obtain sheets with desired properties.

[0098] Figures 13 and 14 show the effects of waviness, as can be seen by the driver of the most demanding automobile. These two figures show images of a lamp reflected by a pressed sheet painted with black paint in a manner comparable to that used in automobile factories. Figure 13 was obtained with a counterexample sheet of the present invention, with a waviness of more than 0.54 μm. Figure 14 is an example of the present invention, with a waviness of less than 0.48 μm. In Figure 13, the waviness has a blurred outline of the reflected image of the lamp, creating a distorted mirror effect. In Figure 14, on the other hand, the outline of the reflected image of the lamp is much clearer. [Explanation of symbols]

[0099] 10 grain boundaries 12 Deformed particles 14 Shade 16 subgrain 22 Recovery Particles 32 Recrystallized particles 32c Elongated recrystallized grains 32d Large equiaxed recrystallized grains 32e Small equiaxed recrystallized grains

Claims

1. The following successive steps: a. in weight percent, i. 0.10% to 0.40% Fe; ii. 0.05% to 0.20% Mn; iii. 0.01% to 0.04% Cr; a process for producing a 6xxx series aluminum alloy comprising: b. Casting of aluminum alloys in plates, preferably by semi-continuous vertical casting; c. Homogenization of the plate at a homogenization temperature of 540°C to 580°C, preferably above 550°C, followed by a preferably forced cooling step to a hot rolling start temperature of 400 to 520°C or to a temperature below the hot rolling start temperature; d. A first hot rolling step from a hot rolling start temperature to a first hot rolling end temperature of 370-450°C; e. A second hot rolling step of the strip from the finish temperature of the first hot rolling to a rolling finish temperature of 250-380°C under conditions such that the microstructure of the strip is either recrystallized or not recrystallized after the second hot rolling; f. Cold rolling of the strip, optionally with annealing in a continuous furnace and without static annealing; g. Solution treatment followed by quenching of the strip into a sheet, preferably in air; h. a pre-aging step, preferably at a pre-aging temperature of 50-120°C for a period of 2-16 hours, suitably obtained by coiling followed by cooling to ambient temperature; i. Aging step of 72 hours to 6 months; A method for manufacturing a metal sheet comprising: A method of manufacturing wherein at least an intermediate recrystallization occurs in steps e and / or f, followed by a cold rolling reduction of at least 60%.

2. The 6xxx series aluminum alloy further comprises, in weight percent: ・Mg: 0.25% to 1.0%, ・Si: 0.30% to 1.5%, preferably Cu≦0.25%, preferably Zn≦0.25%, preferably Ti≦0.15%, preferably V≦0.20%, Preferably, other elements are each ≦0.05% and the total ≦0.15%; The rest is Al 2. The method of claim 1, comprising:

3. The 6xxx series aluminum alloy further comprises, in weight percent: Si: 0.30% to 1.5%, preferably 0.30% to 1.05%, Cu≦0.25%, ・Mg: 0.25% to 0.8%, Zn≦0.25%, Ti≦0.15%, optionally V: 0.05% to 0.20%, Other elements each at a maximum of ≤0.05%, total ≤0.15% The rest is Al 2. The method of claim 1, comprising:

4. The 6xxx series aluminum alloy further comprises, in weight percent: Si: 0.5% to 1.5%, preferably 0.5% to 1.05%, Cu≦0.20%, Mg: 0.25% to 0.6%, Zn≦0.20%, Ti≦0.15%, other elements each ≦0.05%, total ≦0.15%, balance Al; Or, Si: 0.6% to 0.9%, Cu≦0.10%, Mg: 0.40% to 0.6%, Zn≦0.10%, Ti≦0.10%, other elements each ≦0.05%, total ≦0.15%, balance Al, Alternatively, Si: 0.8% to 1.5%, preferably 0.8% to 1.05%, Cu: 0.01% to 0.11%, Mg: 0.45% to 0.7%, Zn≦0.25%, Ti≦0.10%, other elements each ≦0.05%, total ≦0.15%, balance Al; Or, Si: 0.30% to 0.6%, Cu≦0.25%, Mg: 0.40% to 0.8%, Zn≦0.10%, Ti≦0.10%, V: 0.05% to 0.20%, other elements each ≦0.05%, total ≦0.15%, balance Al, Or, Si: 0.50% to 0.9%, Cu: 0.20% to 0.8%, Mg: 0.40% to 0.7%, Zn≦0.20%, Ti≦0.10%, V: 0.05% to 0.20%, other elements each ≦0.05%, total ≦0.15%, balance Al, 2. The method of claim 1, comprising:

5. 5. The method according to claim 1, wherein the Si content of these compositions is at most 1.05% by weight.

6. The 6xxx series aluminum alloy further comprises, in weight percent: ・Si: 0.80% to 1.05%, ・Cu: 0.05% to 0.20%, ・Mg: 0.30% to 0.50%, optionally Zn≦0.25%, ・Ti: 0.01% to 0.10%, Other elements each at a maximum of ≤0.05%, total ≤0.15% The rest is Al 2. The method of claim 1, comprising:

7. 7. The method according to any one of claims 1 to 6, characterized in that the 6xxx series aluminium alloy contains an Na content of not more than 2.5 ppm by weight, preferably not more than 2.0 ppm.

8. 8. The method according to claim 1, wherein production step a) comprises a liquid metal treatment method making it possible to remove all or part of the Na.

9. 9. The method according to claim 1, wherein the cooling in step c) is direct to said temperature.

10. 10. The method according to any one of claims 1 to 9, characterized in that the difference between the hot rolling start temperature and the first hot rolling end temperature is a minimum of -10°C and a maximum of 90°C, preferably 50°C, more preferably 40°C, more preferably 30°C.

11. 11. The method according to any one of claims 1 to 10, characterized in that the total cold reduction is at least 75%.

12. Step d is preferably direct cooling from the homogenization temperature to the start of rolling temperature; The hot rolling start temperature is 450 to 520°C. the end temperature of the hot rolling is less than 330°C, preferably less than 325°C; the microstructure of the strip after step e is not recrystallized; step g includes an intermediate annealing in a continuous furnace to effect recrystallization of the strip; The method according to any one of claims 1 to 11, characterized in that

13. step d is cooling to a temperature below 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; The method according to any one of claims 1 to 11, characterized in that

14. step d is direct cooling from the homogenization temperature to a hot rolling start temperature of 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; The method according to any one of claims 1 to 11, characterized in that

15. Sheet of tempered T4 obtained by the method according to any one of claims 1 to 9.

16. 16. The sheet according to claim 15, characterized in that the recrystallized grains have an average length of 10 to 50 μm, preferably 20 to 40 μm, as measured by ASTM E112-13, and / or the ratio of the length in the longitudinal direction to the thickness in the transverse direction of the recrystallized grains is 2 or less.

17. 17. Sheet according to claim 15 or 16, characterized in that the sheet obtains an elastic limit in the transverse rolling direction of at least 160 MPa and / or at most 260 MPa after a pretraction of 2% in the transverse rolling direction followed by a heat treatment at 185°C for 20 minutes.

18. 18. Sheet according to any one of claims 15 to 17, characterized in that it has a waviness of less than 0.54 μm, preferably less than 0.50 μm, more preferably less than 0.48 μm, measured according to standard SEP 1941 of May 2012.

19. A vehicle body part, preferably visible, obtained by a process comprising shaping, preferably pressing, of a sheet according to any one of claims 15 to 18, followed by paint baking.

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