Method for manufacturing 6xxx alloy sheet with excellent surface quality

Abstract

The invention discloses a manufacturing method of a 6xxx alloy sheet, and the 6xxx alloy sheet is in a T4 state and comprises 0.10% to 0.40% of Fe, 0.05% to 0.20% of Mn and 0.01% to 0.04% of Cr. The method comprises the steps of cooling after homogenization, secondary hot rolling and intermediate recrystallization during cold rolling. The sheet of the present invention achieves a compromise between bending and surface corrugation, resulting in improved post-baking quality.

Description

Technical Field

[0001] The present invention relates to the field of aluminum alloy sheets, and the aluminum alloy sheets are intended to manufacture body components of an automotive white body by stamping. Background Art

[0002] Aluminum alloys are increasingly used in automotive components to reduce the weight of the vehicle, thereby reducing fuel consumption and greenhouse gas emissions.

[0003] Aluminum alloys are particularly used to produce body components, especially visible body components, and more particularly exterior components. Visible body components (especially exterior components) are subject to technical specifications so that these components have an appearance that meets the requirements of automotive manufacturers after painting. Conventionally, the surface quality is characterized by measuring streaks (roping), but this characterization does not reflect all surface defects.

[0004] European application EP1967598 discloses a metal sheet made of a 6000 series aluminum alloy, which contains Si and Mg as main alloy components and has excellent formability sufficient to allow bending in a flat form, excellent dent resistance, and good hardenability during baking. This application discloses a method for manufacturing an aluminum alloy sheet, which includes homogenizing an ingot; cooling the ingot at a cooling rate of 100 °C / h or higher to a temperature below 350 °C, optionally cooling to ambient temperature; reheating the ingot to a temperature of 300 °C to 500 °C and performing hot rolling on it, performing cold rolling on the hot-rolled product; and performing solution heat treatment on the cold-rolled sheet at a temperature of 400 °C or higher, and then quenching.

[0005] European application EP3485055 discloses a method for preparing a 6xxx series aluminum sheet, which includes the following steps: homogenizing a 6xxx series aluminum ingot; directly cooling the homogenized ingot at a cooling rate between 150 °C / h and 2000 °C / h to the starting temperature of hot rolling; hot rolling the ingot to the final hot-rolled thickness and coiling it at the final hot-rolled thickness under the condition of obtaining at least 50% crystallization; performing cold rolling to obtain a cold-rolled sheet. The method of this invention is particularly suitable for manufacturing metal sheets for the automotive industry, and such metal sheets combine high tensile strength suitable for cold stamping operations, good formability, high surface quality, and high corrosion resistance at high productivity.

[0006] International Application WO2018 / 206696 discloses a method for manufacturing an aluminum alloy rolled sheet having excellent formability and good paint baking hardenability, which includes: (a) casting an Al-Si-Mg aluminum alloy ingot, the aluminum alloy containing Si 1.0%-1.50% and Mg 0.10%-0.40% by weight%; (b) heating the ingot to a temperature higher than 550°C; at a temperature higher than 550°C, holding the ingot for at least about 4 hours; cooling the ingot to a temperature of 460°C-520°C; and at a temperature of 460°C-520°C, holding the ingot for less than 6 hours; (c) in one or more rolling steps, hot rolling the ingot into an intermediate gauge of 15 mm-40 mm, wherein the hot rolling mill exit temperature is 370°C-480°C; (d) in one or more rolling steps, further hot rolling from the intermediate gauge into a final hot rolling gauge, wherein the hot rolling mill exit temperature is 310°C-400°C; (e) cooling the hot rolled material in the final hot rolling gauge from the hot rolling mill exit temperature to ambient temperature; (f) cold rolling the hot rolled product into a cold rolled product of the final gauge.

[0007] US Application US20210340654 discloses a method for preparing a 6xxx series aluminum sheet, the method including the steps of: homogenizing an ingot composed of a 6xxx series aluminum alloy, the 6xxx series aluminum alloy containing 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, the balance being aluminum and unavoidable impurities, the various impurities up to 0.05 and in total not exceeding 0.15, hot rolling the ingot on a reversible rolling mill to an exit thickness with a rough hot rolling exit temperature lower than 420°C, finally hot rolling the ingot to a final hot rolling thickness with a tandem rolling mill, coiling at the final hot rolling thickness at a hot rolling exit temperature lower than 300°C, and cold rolling to obtain a cold rolled sheet. The products obtained according to the method of the invention are particularly suitable for automotive hood interior parts because they have the mechanical properties required for pedestrian safety and surface quality. The object of this application is to provide a metal sheet for interior parts (such as hood liners) that are only visible when the vehicle hood is open. Such parts are not subject to the same level of surface quality requirements as external parts because they do not need to compromise on safety in the case of, for example, a pedestrian collision.

[0008] A new requirement has emerged, namely the waviness of the metal sheet surface in the T4 state. If the sheet in the T4 state has excessive waviness, a distorted mirror effect will appear on the painted body parts, which may displease the most demanding motorists.

[0009] Technical problem to be solved

[0010] The technical problem to be solved lies in developing a 6xxx series alloy sheet, which aims to achieve an excellent balance of the following properties:

[0011] The waviness of the sheet in the T4 state is less than or equal to 0.54 μm,

[0012] The bending angle of the sheet in the T4 state after natural aging for 6 months is greater than or equal to 125°. Summary of the invention

[0013] An object of the present invention is to provide a method for manufacturing a rolled sheet made of 6xxx series aluminum alloy, the method comprising the following consecutive steps:

[0014] a. Prepare a 6xxx series aluminum alloy, the 6xxx series aluminum alloy comprising, by weight:

[0015] i. Fe 0.10% to 0.40%,

[0016] ii. Mn 0.05% to 0.20%,

[0017] iii. Cr 0.01% to 0.04%,

[0018] b. Cast the aluminum alloy into a sheet, preferably by semi - continuous vertical casting,

[0019] c. Homogenize the sheet at a homogenization temperature between 540 °C and 580 °C, preferably above 550 °C, and then cool (preferably by forced cooling) to a hot - rolling starting temperature between 400 °C and 510 °C or a temperature below the hot - rolling starting temperature,

[0020] d. Perform a first hot - rolling at a temperature from the hot - rolling starting temperature to a first hot - rolling end temperature of 370 °C to 450 °C,

[0021] e. Perform a second hot - rolling at a temperature from the first hot - rolling end temperature to a rolling end temperature of 250 °C

[0022] to 380 °C to obtain a strip, provided that the microstructure of the strip after the second hot - rolling is recrystallized or non - recrystallized,

[0023] f. Cold - roll the strip (and optionally anneal it in a continuous furnace) without performing static annealing,

[0024] g. Perform solution heat treatment on the strip, preferably in air, to obtain a sheet,

[0025] h. Preferably, pre-aging is carried out at a pre-aging temperature of 50 °C to 120 °C for 2 hours to 16 hours, and preferably the pre-aging is obtained by coiling and subsequent cooling to ambient temperature.

[0026] i. Natural aging is carried out for 72 hours to 6 months.

[0027] Wherein at least intermediate recrystallization occurs in step e and / or f, and subsequently cold rolling is carried out with a reduction of at least 60%.

[0028] Another object of the present invention is to provide a metal sheet obtained by the method according to the present invention.

[0029] Another object of the present invention is to provide a vehicle body part (preferably a visible vehicle body part), which is obtained by a method including forming (preferably stamping) the sheet of the present invention and subsequent painting. Description of the Drawings

[0030] Figure 1 : This figure shows the microstructure of the strip in the T4 state.

[0031] Figure 2 : This figure shows the microstructure of the deformed grains.

[0032] Figure 3 : This figure shows the microstructure of the recovered grains.

[0033] Figure 4 : This figure shows the microstructure of the elongated recrystallized grains.

[0034] Figure 5 : This figure shows the microstructure of the large equiaxed recrystallized grains.

[0035] Figure 6 : This figure shows the microstructure of the small equiaxed recrystallized grains.

[0036] Figure 7 : This figure shows the microstructure of the partially recrystallized or mixed type.

[0037] Figure 8 : This figure shows the microstructure of the recrystallized strip after hot rolling.

[0038] Figure 9 : This figure shows the microstructure of the recrystallized strip after annealing in a continuous furnace.

[0039] Figure 10 : This figure shows the microstructure of the recrystallized strip after static annealing.

[0040] Figure 11: This figure shows examples of level 1, level 2, and level 3 (1 average - 3 excellent) of the looper - line characterization of the specimens.

[0041] Figure 12 : This figure shows the effect of Na on waviness.

[0042] Figure 13 : This figure shows the stamped and painted sheet of the comparative example of the present invention.

[0043] Figure 14 : This figure shows the stamped and painted sheet of the embodiment of the present invention. Detailed Description of the Invention

[0044] Unless otherwise specified, all aluminum alloys discussed hereinafter are named according to the rules and names defined by the "Aluminum Association" in its regularly published "Registration Record Series". Unless otherwise specified, the composition is expressed in weight %. The expression 1.4Cu means that the copper content is 1.4% by weight. For Na, the composition is expressed in weight ppm. The expression 1.4Na means that the sodium content is 1.4 ppm by weight.

[0045] The alloy groups, also known as series, are defined in EN 573 - 1 (2005).

[0046] The metallurgical states discussed are in accordance with the European standard EN 515 (2017).

[0047] The static tensile mechanical properties, namely the ultimate tensile strength Rm, the conventional yield strength Rp0.2 at 0.2% elongation, the reduction of area Ag%, and the elongation at break A%, are determined by a tensile test according to NF EN ISO 6892 - 1 (2018), and the sampling and testing directions are defined by EN 485 - 1 (2009).

[0048] The bending angle, called the α range, is determined by a three - point bending test according to NF EN ISO 7438 (2005) and the procedures of VDA 238 - 100 version 2010 and VDA 239 - 200 version 2017.

[0049] The grain size is measured according to ASTM E112 - 13 (2021).

[0050] Unless otherwise specified, the definitions of EN 12258 - 1 (2012) apply.

[0051] The roping is measured as follows. A strip measured to be approximately 270 mm (in the transverse direction) × 50 mm (in the rolling direction) is cut into sheets. Subsequently, a 15% traction pre-deformation is applied perpendicular to the rolling direction (i.e., the length direction of the strip). Subsequently, the strip is subjected to P800 type sandpaper to reveal the roping. Then the roping is visually evaluated and classified on a scale of 1 (significant roping) to 3 (no roping at all). In Figure 11 Examples of roping corresponding to the 1 to 3 grade values are shown.

[0052] The surface waviness Wsa(1 - 5) is measured using the standard SEP1941 of May 2012. The measurement of the waviness Wsa(1 - 5) is carried out on sheets in the state T4 after 15% deformation in the transverse rolling direction. The waviness is the average of 14 measurements over a span of 30 mm in length, with each measurement length being at least 2.5 mm apart from another measurement length.

[0053] Aluminum and aluminum alloys are polycrystalline materials, and their properties and arrangements can be changed by metal deformation (such as rolling, extrusion, or forging) or by heating (such as annealing). During the deformation process 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 certain planes and directions within each crystal. The occurrence of crystal slip during the plastic deformation process increases the density of dislocations in the material and the rotation of the crystals. The crystal rotation accompanying deformation is one of the reasons for the formation of texture or non-random orientation of crystals (also called grains) in polycrystalline materials. Therefore, dislocations are defects in the crystal grains.

[0054] The microstructure of polycrystalline materials (such as aluminum alloys) varies according to their thermomechanical history. For example, aluminum alloys can have a deformed microstructure after deformation, a recovered microstructure after recovery annealing, and a recrystallized microstructure after recrystallization annealing. A more detailed description is given below. Figure 2 An example of a microstructure including deformed grains is shown. In the example shown, the microstructure 2 includes a plurality of deformed grains 12, each grain having a grain boundary 10. Due to deformation, the internal region of the deformed grains 12 includes a high dislocation density, which is shown by shading 14 in Figure 2 above.

[0055] To reduce the free energy of the deformed material, the deformed material can be annealed. Annealing includes heating the deformed material to a high temperature. Generally, there are two types of annealing for treating aluminum alloys: recovery annealing and recrystallization annealing. In the case of recovery annealing, the aluminum alloy is heated to a temperature such that the grain boundaries of the deformed grains are generally maintained, while the dislocations within the deformed grains move towards a low-energy configuration. These lower-energy configurations within the grains are called sub-grains or cells. Therefore, the grains produced by recovery annealing are usually called recovered grains.Figure 3 An example of a microstructure including recovered grains is shown in . In the example shown, the recovered microstructure 3 includes recovered grains 22. The recovered grains 22 generally have the same grain boundaries 10 as the deformed grains 12, but subgrains 16 are formed within the recovered grains 12 due to the recovery annealing.

[0056] During recrystallization annealing, the aluminum alloy is heated to such a temperature that new grains are produced from the deformed grains 12 and / or from the recovered grains 22. These new grains are called recrystallized grains. Recrystallization annealing results in a material having recrystallized grains. Figure 4 , Figure 5 and Figure 6 An example of a microstructure comprising recrystallized grains is shown in FIG. In the example shown, the microstructure 4 comprises elongated recrystallized grains 32c ( Figure 4 ), microstructure 5 comprises large equiaxed recrystallized grains 32d ( Figure 5 ), and the microstructure 6 comprises small equiaxed recrystallized grains 32e ( Figure 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.

[0057] In some cases, annealing can produce partially recrystallized or mixed materials, examples of which are shown in Figure 7 In the example shown, the partially recrystallized or mixed microstructure 7 includes a mixture of recovered grains 22 and recrystallized grains 32 .

[0058] The ambient temperature is any temperature suitable for human activities, ranging from 5 to 35°C.

[0059] method

[0060] The invention is based on the applicant's observation that, with a suitable composition and manufacturing process, it is possible to produce sheets having excellent surface quality after painting while maintaining excellent bending properties. The method is preferably used for sheets for visible body parts, in particular exterior parts.

[0061] The method for making the sheet of the present invention comprises preparing a 6xxx series aluminum alloy comprising, by weight:

[0062] Fe 0.10% to 0.40%,

[0063] Mn 0.05% to 0.20%,

[0064] Cr 0.01% to 0.04%,

[0065] Mn and Cr precipitate during the manufacturing method. Mn and Cr preferably precipitate in the form of a dispersion phase, which is a small precipitate relative to the grain size, with a typical average size of 0.1 to 0.3 μm. These dispersion phases contribute to controlling the various recrystallizations that occur during the manufacturing method, and the recrystallizations affect the waviness of the sheet. Fe also precipitates and also contributes to controlling the various recrystallizations. The contents of these elements are a compromise that matches the manufacturing method.

[0066] If the contents of Mn and Cr are excessive, recrystallization will not occur properly during the manufacturing method, resulting in deteriorated waviness. The maximum content 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 content of Cr is 0.04%; preferably 0.03%, more preferably 0.02%. If the contents of these elements are insufficient, the alloy tends to crystallize in the form of large grains during solution heat treatment, which will deteriorate the waviness. The minimum content of Mn is 0.05%. The minimum content of Cr is 0.01%.

[0067] Too low an Fe content makes the alloy particularly expensive. The addition of Fe that forms insoluble precipitates also contributes to controlling recrystallization, but excessive Fe will deteriorate the bending of the sheet in the T4 condition. A high maximum Fe content is beneficial for making the alloy tolerate the use of recycled products during the production of the alloy of the present invention. Fe is at least 0.10%; preferably 0.15%, more preferably 0.19%. Fe is at most 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%.

[0068] According to EN 573-1 (2005), in the 2xxx to 8xxx groups, the naming of the alloy is determined by the addition element with the highest average percentage present (Mg2Si for 6xxx alloys). If multiple addition elements meet the requirement of the highest average percentage, they are selected in the order of Cu, Mn, Si, Mg, Mg2Si, Zn or others. Preferably, the 6xxx series of alloys contain 0.25% to 1.0% of Mg and 0.30% to 1.5% of Si. More preferably, the 6xxx series of aluminum alloys also contain Cu ≤ 0.25%, Zn ≤ 0.25%, Ti ≤ 0.15%, V ≤ 0.20%, other elements each ≤ 0.05% and in total ≤ 0.15%, with the balance being Al.

[0069] Preferably, Si is at most 1.05% to improve the surface quality characterized by the Romping line.

[0070] In one embodiment, the 6xxx series aluminum alloy further comprises, by weight%:

[0071] Si: 0.30% - 1.5%, preferably 0.30% - 1.05%,

[0072] Cu ≤ 0.25%,

[0073] Mg 0.25% - 0.8%,

[0074] Zn ≤ 0.25%,

[0075] Ti ≤ 0.15%,

[0076] Optionally V 0.05% - 0.20%,

[0077] Each of the other elements ≤ 0.05% and in total ≤ 0.15%,

[0078] with the balance being aluminum.

[0079] In one embodiment, the 6xxx series aluminum alloy further comprises, by weight%:

[0080] Si: 0.5% - 1.5%, preferably 0.5% - 1.05%, Cu ≤ 0.20%, Mg 0.25% - 0.6%, Zn ≤ 0.20%, Ti ≤ 0.15%, each of the other elements ≤ 0.05% and in total ≤ 0.15%, with the balance being Al,

[0081] Or Si: 0.6% - 0.9%, Cu ≤ 0.10%, Mg 0.40% - 0.6%, Zn ≤ 0.10%, Ti ≤ 0.10%, each of the other elements ≤ 0.05% and in total ≤ 0.15%, with the balance being Al, or Si: 0.8% - 1.5%, preferably 0.8% - 1.05%, Cu 0.01% - 0.11%, Mg

[0082] 0.45% - 0.7%, Zn ≤ 0.25%, Ti ≤ 0.10%, each of the other elements ≤ 0.05% and in total ≤ 0.15%, with the balance being Al,

[0083] 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%, each of the other elements ≤ 0.05% and in total ≤ 0.15%, with the balance being Al,

[0084] 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%, and other elements each ≤ 0.05%

[0085] and in total ≤ 0.15%, with the balance being Al.

[0086] Preferably, the Si content is at most 1.05% to improve the Luo Ping line.

[0087] Preferably, the 6000 - series alloy has the following element composition, in wt%, excluding Fe, Mn, and Cr:

[0088] Si: 0.80% - 1.05%,

[0089] Cu: 0.05% - 0.11%,

[0090] Mg 0.30% - 0.50%,

[0091] Optionally Zn ≤ 0.25%,

[0092] Ti 0.01% - 0.10%

[0093] Other elements each ≤ 0.05% and in total ≤ 0.15%,

[0094] with the balance being aluminum.

[0095] Preferably, the Si content in this composition is at most 1.05% to improve the Luo Ping line.

[0096] Si and Mg form Mg2Si precipitates, enabling mechanical properties to be obtained after baking. Excess Si (subtracting the Mg content from the Si content) improves formability in the T4 condition.

[0097] Cu is beneficial to the mechanical properties in the T4 and T6 conditions. Excess Cu may damage the corrosion resistance. Preferably, Cu is at most 0.15%, more preferably 0.10%.

[0098] Zn can be optionally added to facilitate recycling, but should not be added in excess to avoid corrosion phenomena.

[0099] Ti has the effect of refining the grain size. The maximum Ti content is preferably 0.10%.

[0100] In an embodiment of each of the alloys, the Na content is less than or equal to 2.5 ppm, preferably 2.0 ppm. Controlling the maximum Na content helps control the waviness. A low Na content can be achieved by using very high purity aluminum and the additive elements required to produce the alloy. A more economical solution is to include in the process of preparing the 6xxx series aluminum alloy a process for treating the liquid metal to remove all or part of the Na. Non-limiting examples of processes for removing all or part of the Na are taught in International Application WO2022 / 242992. Preferably, Na is at least 0.3 ppm, more preferably 0.5 ppm. This minimum value is a relevant compromise between waviness and productivity and cost constraints.

[0101] The aluminum alloy is then cast into a sheet, preferably by vertical semi-continuous casting (direct chill casting or DC casting). The preferred dimensions of the sheet of the present invention are a thickness of 200 mm to 600 mm, a width of 1000 mm to 3000 mm, and a length of 2000 mm to 8000 mm. Vertical semi-continuous casting can obtain a sheet structure that is more uniform than that obtained by continuous casting.

[0102] Advantageously, the sheet produced is integral. This is more economical than a plated sheet.

[0103] The sheet is then homogenized at a homogenization temperature between 540 °C and 580 °C, preferably above 550 °C, and then cooled (preferably forced cooling) to a hot rolling starting temperature between 400 °C and 520 °C or a temperature below the hot rolling starting temperature. The homogenization temperature exceeds the solid solution temperature of the alloy while avoiding local melting and burning. The homogenization temperature is preferably at most 580 °C, preferably 570 °C, and at least 540 °C, preferably at least 550 °C. Too high or too low a temperature deteriorates the mechanical properties of the sheet after aging. Insufficient homogenization temperature or duration may deteriorate the waviness because Mg2Si is not sufficiently solid-solved, so that its subsequent precipitates do not have an optimal size. The duration of homogenization is preferably greater than 1 hour. Too short a homogenization duration deteriorates the mechanical properties of the sheet after aging.

[0104] After the sheet is homogenized, the sheet is cooled (preferably forced cooling) to a hot rolling starting temperature of 400 °C to 510 °C or a temperature below the hot rolling starting temperature. Advantageously, the cooling is direct cooling (i.e., no secondary intermediate platform is provided during homogenization) to a hot rolling starting temperature of 400 °C to 510 °C or a temperature below the hot rolling starting temperature, so as not to reduce productivity. Cooling the sheet after homogenization can obtain precipitates of Mg2Si with optimal size, so that the necessary recrystallization can be controlled in subsequent steps to obtain the required waviness. Direct hot rolling at the homogenization temperature for reasons of productivity will result in precipitates of Mg2Si, which will trigger recrystallization causing coarse grains in subsequent steps, thus deteriorating the waviness. Preferably, forced cooling is carried out to avoid excessive growth of Mg2Si precipitates, which is unfavorable for the solution heat treatment of Mg2Si and thus damages the mechanical properties. Excessive growth of Mg2Si precipitates deteriorates the waviness.

[0105] In one embodiment, in order not to reduce productivity, the homogenized sheet is directly cooled to the hot rolling starting temperature. The cooling is preferably forced cooling at a direct cooling rate of at least 150 °C / h. Advantageously, the direct cooling rate is at most 500 °C / h. The cooling can typically be implemented by a machine such as described in International Application WO2016012691. The minimum rate of 150 °C / h is a compromise between precipitation kinetics and productivity. A cooling rate exceeding 500 °C / h will result in non-uniform temperature inside the sheet, which will lead to non-uniform precipitates of Mg2Si and thus may result in a mixed microstructure in the subsequent recrystallization process. The mixed microstructure results in unacceptable waviness.

[0106] In another embodiment, the homogenized sheet is cooled and then reheated to the hot rolling starting temperature. Preferably, the cooling is then directly reheated to the hot rolling starting temperature so as not to reduce productivity. Preferably, the cooling is forced cooling with a fan that blows air at ambient temperature onto the homogenized sheet, thus cooling faster than natural cooling at ambient temperature. The cooling is preferably carried out to a temperature below 300 °C at a rate of preferably 60 °C / h to 120 °C / h, more preferably 70 °C / h to 90 °C / h. Preferably, the sheet is allowed to complete natural cooling to ambient temperature. Continuing to cool below the hot rolling starting temperature is advantageous because it can continue the precipitation kinetics of Mg2Si, thus facilitating recrystallization in subsequent steps required for the waviness. Then the sheet is heated to the hot rolling starting temperature.

[0107] Perform the first hot rolling at a temperature ranging from the starting temperature of the hot rolling to the ending temperature of the first hot rolling, which is 370°C to 450°C. The first hot rolling is preferably continuously carried out on a rolling mill, two or more reversing hot rolling mills. The ending thickness of the first hot rolling is 30 - 50 mm. The first hot rolling is preferably carried out in such a way that the sheet is not heated during each pass of the hot rolling. Preferably, the cooling between the starting temperature of the hot rolling and the ending temperature of the first hot rolling is at most 90°C, preferably 50°C, more preferably 40°C, even more preferably 30°C. Preferably, the difference between the starting temperature of the hot rolling and the ending temperature of the first hot rolling is a positive number or zero. However, during the first hot rolling, a slight reheating of 10°C, i.e., the minimum difference between the starting temperature of the hot rolling and the ending temperature of the first hot rolling is -10°C, is acceptable. This hot rolling method enables the control of the continuation of Mg2Si precipitates, especially when the sheet has been directly cooled to the hot rolling temperature. The first hot rolling temperature range helps to control the recrystallization in subsequent manufacturing steps. Limiting the maximum cooling during the first hot rolling further simplifies the method as it helps to eliminate the need for subsequent intermediate annealing.

[0108] Next, perform the second hot rolling at a temperature ranging from the ending temperature of the first hot rolling to the ending temperature of the rolling, which is 250°C to 380°C, to obtain a strip, provided that the microstructure of the strip after the second hot rolling is recrystallized or non-recrystallized. That is, the microstructure does not include a recrystallized zone and a non-recrystallized zone. A mixed microstructure after hot rolling will disrupt the subsequent recrystallization and deteriorate the waviness of the T4 state sheet. The second hot rolling is preferably carried out on a tandem hot rolling mill including 2, 3, 4, 5, 6 or more hot rolling mills. Then coil the obtained strip. Preferably, the coil is naturally cooled to the ambient temperature. Preferably, forced cooling or quenching is not carried out during the second hot rolling. Preferably, the microstructure is characterized after the strip is cooled to the ambient temperature.

[0109] In one embodiment, the ending temperature of the hot rolling is higher than 330°C, preferably higher than 340°C, more preferably higher than 350°C. The high temperature at the end of the hot rolling enables the obtaining of medium-grained recrystallization, which is beneficial for the final recrystallization to obtain a waviness less than 0.50 μm. The high temperature simplifies the manufacturing method by eliminating the need for intermediate annealing.

[0110] In another embodiment, the ending temperature of the hot rolling is lower than 330°C, preferably lower than 325°C, more preferably lower than 320°C. The low temperature at the end of the hot rolling makes it possible to obtain a non-recrystallized microstructure, which is beneficial for obtaining subsequent recrystallization to obtain a waviness less than 0.50 μm.

[0111] Then the strip is cold rolled (and optionally intermediate annealed on a continuous furnace), without performing static annealing. The annealing is intermediate annealing, where the intermediate annealing is before and after cold rolling. Static annealing is carried out in a furnace, where the coiled strip is annealed and heat treated. Static annealing is not carried out before, during or after the cold rolling step. Static annealing cannot achieve a compromise between surface quality and bendability because the duration of this heat treatment results in large grains.

[0112] Preferably, the total cold rolling reduction ratio is at least 75%. The total cold rolling reduction ratio is the reduction ratio between the thickness at the end of hot rolling and the final thickness. The said reduction ratio helps to obtain a waviness less than or equal to 0.50 μm.

[0113] Preferably, the annealing on the continuous furnace is recrystallization annealing. In one embodiment, the annealing on the continuous furnace is carried out at a PMT (peak metal temperature) below the alloy solution temperature. In another embodiment, the annealing on the continuous furnace also performs solution heat treatment at a temperature higher than the solution temperature and lower than the combustion temperature. Preferably, a high PMT is selected to minimize the duration above 350 °C, so as to obtain recrystallization with medium grains. Recrystallization on the continuous furnace means that cold rolling is carried out before solution heat treatment.

[0114] The thickness of the strip after cold rolling is 0.8 mm to 1.2 mm. Since the body parts do not have sufficient rigidity, too thin a thickness cannot be used. Too thick a thickness will cause the body parts to be too heavy to be used.

[0115] At least one intermediate recrystallization is carried out in the second hot rolling and / or cold rolling step, followed by cold rolling with a reduction of at least 60%. Recrystallization occurs in the second hot rolling step, where the microstructure of the strip after hot rolling is recrystallized. The recrystallization in the cold rolling step is preferably obtained by annealing on a continuous furnace. Cold rolling is carried out after intermediate recrystallization. The reduction ratio of at least 60% deforms the recrystallized grains so as to obtain fine-grained recrystallization during the solution heat treatment necessary for waviness. The said reduction ratio of at least 60% is the reduction ratio between the intermediate recrystallization thickness and the final thickness.

[0116] The lack of intermediate recrystallization in the present invention will lead to insufficient bending or a surface quality characterized by the Luo Ping line being not suitable for visible body parts. The combination of the intermediate recrystallization of the present invention with cold rolling between the intermediate recrystallization and the solution heat treatment enables the control of the final recrystallization occurring during the solution heat treatment and the obtaining of the required waviness. Preferably, the medium grains obtained by this intermediate recrystallization are elongated grains, and their length dimension in the long rolling direction is 70 μm to 200 μm, as Figure 9As shown. 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 transverse direction. The recrystallized grains obtained after hot rolling have this size at half thickness, as Figure 8 shown. Overly long grains leave relic features at the grain boundaries resulting from the final recrystallization carried out by solution heat treatment, and such relic features will lead to inappropriate waviness. This is the case for the grains obtained by static annealing crystallization, as Figure 11 shown.

[0117] The cold rolling reduction rate between the intermediate recrystallization and the solution heat treatment is at least 60% so that the strip can be fully deformed during the solution heat treatment necessary for obtaining waviness to achieve the final recrystallization.

[0118] Then, the sheet is subjected to solution heat treatment at a solution heat treatment temperature higher than the alloy solution temperature while avoiding local melting or burning, and then quenched, preferably quenched in a continuous furnace. The solution heat treatment temperature is preferably at most 580 °C, preferably 570 °C; and at least 540 °C, preferably 550 °C. Undercooled solution heat treatment and / or overly short solution heat treatment will reduce the mechanical properties of the sheet due to insufficient solution heat treatment. Overheated solution heat treatment results in primary melting and reduces the mechanical properties. An overly long solution heat treatment time will reduce the productivity.

[0119] The solution heat treatment also causes the final recrystallization. Preferably, the final recrystallization is a recrystallization with fine grains having an average length of 10 to 50 μm, as Figure 10 shown. Preferably, the aspect ratio of the recrystallized grains is not greater than 2, as Figure 10 shown. The length of the grains is measured along the rolling direction, and the thickness of the grains is measured along the short transverse direction. Longer or finer grains are not conducive to waviness. This microstructure will not be changed by pre-aging and aging.

[0120] The solution heat treatment temperature is preferably at least 540 °C, preferably 550 °C; at most 570 °C. Preferably, it is not maintained at the solution heat treatment temperature to avoid grain coarsening.

[0121] Quenching is preferably carried out in air to limit flatness defects.

[0122] Preferably, the sheet is pre-aged. Preferably, the sheet is reheated to be pre-aged for 2 to 16 hours at a pre-aging temperature of 50°C to 120°C, preferably 65°C to 90°C. Heating is useful when the sheet is subjected to a surface treatment at a temperature lower than the pre-aging temperature between quenching and pre-aging. Preferably, pre-aging is obtained by heating without maintaining at the pre-aging temperature, then 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 yield strength in the T4 state and the yield strength after paint baking.

[0123] The sheet is aged at ambient temperature for 72 hours to 6 months to reach the T4 state. This step is a limitation related to the storage before forming. After that, the sheet is in the T4 state.

[0124] The sheet of the present invention can be obtained by the method of the present invention.

[0125] Preferably, the sheet of the present invention is characterized in that the average size of the recrystallized grains is 10 to 50 μm in length, preferably 20 μm to 40 μm, and / or the ratio of the length of the recrystallized grains in the longitudinal direction to the thickness in the short transverse direction is less than or equal to 2.

[0126] Preferably, the sheet of the present invention is characterized in that the waviness is less than 0.54 μm, preferably 0.50 μm, more preferably 0.48 μm. Reducing the waviness can improve the surface quality after painting.

[0127] Preferably, the sheet of the present invention obtains a yield strength of at least 160 MPa, preferably at least 170 MPa, more preferably at least 180 MPa, more preferably at least 190 MPa, more preferably at least 195 MPa in the rolling transverse direction after simulated paint baking (bake hardening). Too low a yield strength makes the part sensitive to indentation, i.e., sensitive to the impact of hail. Preferably, the yield strength is lower than 260 MPa, preferably 250 MPa, more preferably 240 MPa, more preferably 230 MPa. Too high a yield strength reduces the bendability of the component in the event of an accident. After 2% pre-stretching, simulated paint baking is carried out in the rolling transverse direction, and then heat treatment is carried out at 185°C for 20 minutes.

[0128] The sheet of the present invention can be used to manufacture body components on a vehicle, preferably visible body components. The manufacturing method includes forming, preferably by stamping, and then paint baking. The paint baking known to those skilled in the art corresponds to heat treatment at a temperature of 170°C to 195°C for 10 to 30 minutes.

[0129] Embodiment 1

[0130] In the first embodiment, the homogenized sheet is directly cooled to the starting temperature of hot rolling according to the above description.

[0131] The starting temperature of hot rolling is 450 to 520 °C, preferably 470 to 510 °C, and more preferably 470 to 400 °C. The ending temperature of hot rolling is lower than 330 °C, preferably lower than 325 °C, and more preferably lower than 320 °C. Excessively high temperatures are likely to cause recrystallization. Excessively low temperatures will result in excessive hot rolling stress. The said temperature range enables a non-recrystallized microstructure of the strip to be obtained after hot rolling, and this structure obtains ripples through medium-grain recrystallization during intermediate annealing.

[0132] Intermediate annealing is carried out on a continuous furnace during the cold rolling step. The said intermediate annealing is recrystallization consistent with the above description. The combination of the non-recrystallized structure after hot rolling and annealing on a continuous furnace enables finer grains to be obtained compared to the recrystallized structure after hot rolling.

[0133] Embodiment 2

[0134] In the second embodiment, the homogenized sheet is cooled according to the above description and then reheated to the starting temperature of hot rolling.

[0135] The starting temperature of hot rolling is lower than 450 °C, preferably 440 °C, more preferably 430 °C, and even more preferably 420 °C. Limiting the starting temperature of hot rolling enables the cooling during the first hot rolling to be reduced. The ending temperature of hot rolling is higher than 330 °C, preferably higher than 340 °C, and more preferably higher than 350 °C. The microstructure of the strip after hot rolling recrystallizes with medium grains. The higher ending temperature of hot rolling improves the intermediate recrystallization and then the final recrystallization, thereby obtaining a waviness less than 0.50 μm.

[0136] Step g preferably does not include intermediate annealing on a continuous furnace, which is particularly advantageous for productivity because continuous furnaces are often expensive, custom-sized equipment for the production capacity of a factory with one available continuous furnace.

[0137] The cold rolling reduction ratio is preferably greater than 75%, preferably greater than 80%, to improve recrystallization and thus obtain better waviness. The cold rolling reduction ratio is the reduction ratio between the ending thickness after hot rolling and the final thickness.

[0138] Embodiment 3

[0139] In the third embodiment, the homogenized sheet is directly cooled to the starting temperature of hot rolling according to the above description.

[0140] The starting temperature of hot rolling is below 450 °C, preferably 440 °C, more preferably 430 °C, and even more preferably 420 °C. Limiting the starting temperature of hot rolling enables the reduction of cooling during the first hot rolling. The ending temperature of hot rolling is above 330 °C, preferably above 340 °C, and more preferably above 350 °C. The microstructure of the strip after hot rolling recrystallizes with medium-sized grains. A higher ending temperature of hot rolling improves intermediate recrystallization and then final recrystallization, thereby obtaining a waviness of less than 0.50 μm.

[0141] Step g preferably does not include intermediate annealing on a continuous furnace, which is particularly advantageous for productivity because continuous furnaces are often expensive, custom-sized equipment in terms of the production capacity of a factory with one available continuous furnace.

[0142] The cold rolling reduction ratio is preferably greater than 75%, preferably greater than 80%, to improve recrystallization and thereby obtain better waviness.

[0143] Examples

[0144] The present disclosure is further illustrated by the following examples. These examples are only intended to illustrate the invention and not to limit it.

[0145] Plates of various compositions are cast according to the alloys in Table 1. These plates are cast by semi-continuous vertical casting. The examples of the present invention correspond to plates A, B, C, D, E, F, M, N, O, and P.

[0146] [Table 1]

[0147] Si (%) Fe (%) Cu (%) Mn (%) Mg (%) Cr(％) Ti (%) Na (ppm) A 0.92 0.21 0.09 0.14 0.42 0.02 0.03 0.3 B 0.92 0.21 0.09 0.14 0.41 0.01 0.03 1.0 C 0.91 0.29 0.09 0.14 0.43 0.02 0.03 0.4 D 0.93 0.19 0.09 0.07 0.41 0.01 0.03 1.4 E 0.93 0.19 0.09 0.07 0.41 0.01 0.03 1.0 F 0.89 0.26 0.09 0.17 0.41 0.04 0.02 Not measured G 1.30 0.14 0.08 0.07 0.32 0.01 0.03 1.5 H 0.94 0.20 0.09 0.08 0.42 0.01 0.03 0.8 I 0.92 0.21 0.09 0.14 0.41 0.01 0.03 1.0 J 1.07 0.23 0.09 0.14 0.44 0.04 0.03 0.0 K 0.90 0.24 0.09 0.17 0.41 0.04 0.03 Not measured L 0.95 0.28 0.04 0.18 0.45 0.03 0.03 2.0 M 0.89 0.26 0.09 0.17 0.41 0.04 0.02 1.4 N 0.91 0.17 0.09 0.07 0.41 0.01 0.03 2.6 O 0.90 0.19 0.09 0.07 0.40 0.01 0.04 1.9 P 0.89 0.19 0.09 0.08 0.40 0.01 0.03 4.1

[0148] These plates except plates D and E are homogenized at a temperature of 560 °C for 3 hours, and plates D and E are homogenized at a temperature of 560 °C for 12 hours. Plates D and E are cooled to 300 °C at a cooling rate of 87 °C / h with a fan, then plates D and E are cooled to ambient temperature, and then heated to the starting temperature of hot rolling. The other plates are directly cooled to the starting temperature of hot rolling at a cooling rate of 150 to 500 °C / h.

[0149] Subsequently, the first hot rolling is carried out on a reversible hot rolling mill. The starting temperature of hot rolling and the ending temperature of the first hot rolling are given in Table 2. The thickness after the first hot rolling is between 30 mm and 50 mm. Next, the second hot rolling is carried out on a tandem rolling mill with 4 rolling mills. The ending temperature of hot rolling is as shown in Table 2. Then the coiled strip is naturally cooled to ambient temperature. Next, the microstructure is analyzed. The strips A, B, C, D, E, H, M, N, O, and P after hot rolling undergo recrystallization. The grains are elongated, with a length of 70 μm to 200 μm in the long rolling direction. The ratio of the length to the thickness of the recrystallized grains is 2 to 5. The strips F, G, I, J, K, and L do not undergo recrystallization after hot rolling.

[0150] [Table 2]

[0151] Initial hot rolling temperature (°C) End temperature of the first hot rolling Hot rolling end temperature (°C) Hot rolling end thickness [mm] A 409 402 365 6 B 410 390 353 6 C 403 403 374 6.5 D 403 380 / 400 353 5 E 400 380 / 400 347 4 F 470-490 373 280-320 5 G 483 402 300 6.2 H 408 368 368 6.5 I 477 371 309 7.3 J 470-490 458 280-320 2.8 K 470-490 380 / 400 280-320 7.3 L 470-490 380 / 400 280-320 7.3 M 404 399 359 6.0 N 401 405 368 6.0 O 400 404 367 6.0 P 402 407 367 6.0

[0152] Then, cold-rolled strips are processed according to Table 3, which also lists strips that have undergone intermediate recrystallization annealing. The microstructure of the strips that have undergone intermediate recrystallization annealing is analyzed. Strip F undergoes recrystallization after intermediate annealing. The grains of strip F are elongated, with lengths in the long rolling direction ranging from 70 μm 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 undergo recrystallization, and their grains are larger than 200 μm.

[0153] [Table 3]

[0154]

[0155] Next, solution heat treatment is carried out on the strips at a temperature of 560 °C (PMT). Strips A, B, C, D, E, F, M, N, O, and P undergo recrystallization, with fine grains having an average size ranging from 10 μm to 50 μm and a length-to-thickness ratio not exceeding 2.

[0156] Next, the sheets are pre-aged and then aged for 180 days and are in the T4 state. The sheets thus obtained are characterized, and the results are listed in Table 4. The bending T.T corresponds to the direction Q-Q in the cited standard. The yield strength is measured under a pre-tension of 2% in the rolling transverse direction and heat-treated at 185 °C for 20 minutes to simulate the effect of baking paint.

[0157] [Table 4]

[0158]

[0159] This manufacturing method can obtain sheets with desired properties.

[0160] Figure 13 and 14 shows the effect of waviness observable by the most demanding automotive manufacturers. These two figures show the light reflection images of the stamped sheets, and the stamped sheets are sprayed with black paint in a manner similar to that implemented in an automotive factory. Figure 13 is obtained from the sheet that is a comparative example of the present invention, in which the waviness is greater than 0.54 μm. Figure 14 is an embodiment of the present invention, in which the waviness is less than 0.48 μm. In Figure 13 , the contour of the light reflection image becomes blurred due to the influence of the waves, and the waves have a deformed mirror effect. On the other hand, in Figure 14 , the contour of the light reflection image is much clearer.

Claims

1. A method for manufacturing a metal sheet, the method comprising the following successive steps: a. Prepare an Al alloy of the 6xxx series, the Al alloy of the 6xxx series comprising, by weight: i. Fe 0.10% to 0.40%, ii. Mn 0.05% to 0.20%, iii. Cr 0.01% to 0.04%, b. Cast the Al alloy into a sheet, preferably by semi - continuous vertical casting, c. Homogenize the sheet at a homogenization temperature between 540 °C and 580 °C, preferably above 550 °C, and then cool (preferably by forced cooling) to a hot - rolling starting temperature between 400 °C and 520 °C or a temperature below the hot - rolling starting temperature, d. Carry out a first hot - rolling at a temperature from the hot - rolling starting temperature to a first hot - rolling end temperature of 370 °C to 450 °C, e. Carry out a second hot - rolling at a temperature from the first hot - rolling end temperature to a rolling end temperature of 250 °C to 380 °C to obtain a strip, provided that the microstructure of the strip after the second hot - rolling is recrystallized or non - recrystallized, f. Carry out cold rolling on the strip (and optionally anneal on a continuous furnace), without carrying out static annealing, g. Carry out solution heat treatment on the strip, preferably in air, to obtain a sheet, h. Preferably, pre - age for 2 to 16 hours at a pre - aging temperature of 50 °C to 120 °C, Preferably obtain pre - aging by coiling and subsequent cooling to ambient temperature, i. Carry out natural aging for 72 hours to 6 months, wherein at least intermediate recrystallization occurs in step e and / or f, followed by cold - rolling with a reduction of at least 60%.

2. The method according to claim 1, wherein The Al alloy of the 6xxx series further comprises, by weight%: Mg: 0.25% - 1.0%, Si: 0.30% - 1.5%, Preferably Cu ≤ 0.25%, Preferably Zn ≤ 0.25%, Preferably Ti ≤ 0.15%, Preferably V ≤ 0.20%, Preferably each of the other elements ≤ 0.05% and in total ≤ 0.15%, with the balance being aluminum.

3. The method according to claim 1, wherein The Al alloy of the 6xxx series further comprises, by weight%: Si: 0.30% - 1.5%, preferably 0.30% - 1.05%, Cu ≤ 0.25%, Mg 0.25% - 0.8%, Zn ≤ 0.25%, Ti ≤ 0.15%, Optionally V 0.05% - 0.20%, Each of the other elements ≤ 0.05% maximum and in total ≤ 0.15%, with the balance being aluminum.

4. The method according to claim 1, characterized in that, The 6xxx series alloy further comprises, by weight%: Si: 0.5% - 1.5%, preferably 0.5% - 1.05%, Cu ≤ 0.20%, Mg 0.25% - 0.6%, Zn ≤ 0.20%, Ti ≤ 0.15%, each of the other elements ≤ 0.05% and in total ≤ 0.15%, with the balance being Al, or Si: 0.6% - 0.9%, Cu ≤ 0.10%, Mg 0.40% - 0.6%, Zn ≤ 0.10%, Ti ≤ 0.10%, each of the other elements ≤ 0.05% and in total ≤ 0.15%, with the balance being Al, or Si: 0.8% - 1.5%, preferably 0.8% - 1.05%, Cu 0.01% - 0.11%, Mg 0.45% - 0.7%, Zn ≤ 0.25%, Ti ≤ 0.10%, other elements each ≤ 0.05% and in 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% and in total ≤ 0.15%, the balance being 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% and in total ≤ 0.15%, the balance being Al.

5. The method according to any one of the preceding claims, characterized in that, The Si content in its composition is at most 1.05% by weight.

6. The method according to claim 1, wherein The 6xxx series alloy further comprises, 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% maximum and in total ≤ 0.15%, the balance being aluminum.

7. The method according to any one of the preceding claims, characterized in that, The 6xxx series alloy contains ≤ 2.5 weight ppm of Na, preferably ≤ 2.0 weight ppm.

8. The method according to any one of the preceding claims, characterized in that, Manufacturing step a includes a process of treating the liquid metal to remove all or part of the Na.

9. The method according to any one of the preceding claims, characterized in that The cooling in step c is direct cooling to the said temperature.

10. The method according to any one of the preceding claims, characterized in that, The difference between the starting temperature of the hot rolling and the ending temperature of the first hot rolling is at least -10°C and at most 90°C, preferably 50°C, more preferably 40°C, even more preferably 30°C.

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

12. The method according to any one of the preceding claims, characterized in that, step d is preferably direct cooling from the homogenization temperature to the starting temperature of the hot rolling, the starting temperature of the hot rolling is 450°C to 520°C, the ending temperature of the hot rolling is lower than 330°C, preferably lower than 325°C, the microstructure of the strip after step e is non-recrystallized, step g includes intermediate annealing on a continuous furnace to recrystallize the strip.

13. The method according to any one of claims 1 to 11, characterized in that, step d is cooling to a temperature lower than the starting temperature of the hot rolling, and the starting temperature of the hot rolling is lower than 450°C, the ending temperature of the second hot rolling is higher than 340°C, preferably higher than 350°C, the microstructure of the strip after step e is recrystallized, step g preferably does not include intermediate annealing.

14. The method according to any one of claims 1 to 11, characterized in that, step d is direct cooling from the homogenization temperature to the starting temperature of the hot rolling, and the starting temperature of the hot rolling is lower than 450°C, the ending temperature of the second hot rolling is higher than 340°C, preferably higher than 350°C, the microstructure of the strip after step e is recrystallized, step g preferably does not include intermediate annealing. A sheet in the T4 state obtained by the method according to any one of claims 1 to 9.

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

17. The sheet according to claim 15 or 16, characterized in that, After the sheet is pre-stretched by 2% in the rolling transverse direction and then heat-treated at 185 °C for 20 minutes, a yield strength of at least 160 MPa and / or not greater than 260 MPa is obtained in the rolling transverse direction.

18. The sheet according to any one of claims 15 to 17, characterized in that, The waviness measured by the standard SEP1941 in May 2012 is less than 0.54 μm, preferably less than 0.50 μm, and more preferably less than 0.48 μm.

19. A vehicle body part, preferably a visible vehicle body part, obtained by a method comprising forming (preferably stamping) a sheet according to any one of claims 15 to 18 and then painting.

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