Improved method for manufacturing structural components for motor vehicle bodies

Abstract

A method for producing rolled products for automotive bodies or body structures using an alloy containing 0.75-1.10 Si, up to 0.4 Fe, 0.5-0.8 Cu, 0.1-0.4 Mn, 0.75-1 Mg, up to 0.15 Ti, up to 0.1 Cr, and up to 0.1 V is disclosed, with multiple process steps ranging from casting the ingot to forming and painting the body part. Various possibilities for pre-aging the sheet and heat treating the part provide advantageous as-formed material properties, material strength, and low susceptibility to bake hardening processes, which may vary depending on the location of the part within the body.

Description

[Technical Field]

[0001] The invention particularly relates to the field of structural parts or components of motor vehicles, also called "body in white", such as in particular impact absorbers or "crash boxes", reinforcing parts, linings or other body structural parts, manufactured by stamping aluminium alloy sheet, more particularly alloys within the AA6xxx series according to the Aluminum Association designation, which are intended to irreversibly absorb energy upon impact and which have an excellent compromise between good crash behaviour and high mechanical strength.

[0002] More specifically, the present invention relates to the manufacture of such components by stamping in a solution hardened, quenched and naturally aged temper, followed by hardening by aging and baking the paint on the part, or "bake hardening." [Background technology]

[0003] Aluminum alloys are increasingly used in automobile manufacturing to reduce vehicle weight and thus reduce fuel consumption and greenhouse gas emissions.

[0004] Aluminium alloy sheets are used in particular to manufacture many parts of the "body in white", such as body skin parts (or outer panels of the body), for example the front wings, roof, bonnet, trunk or door skins, and lining parts or body structural components, such as doors, bonnet, tailgate or roof linings or reinforcements, or spars, bulkheads, load floors, floor tunnels and front, middle and rear pillars, and finally impact absorbers or "crash boxes".

[0005] Even though many skin components are already produced from aluminum alloy sheets, the replacement of steel with aluminum for lining or structural components with complex geometries has proven to be more challenging, firstly because aluminum alloys are not as formable as steels, and secondly because they generally have lower mechanical properties than the steels used for this type of component.

[0006] The reason is that this type of application requires the following set of sometimes contradictory properties: - High formability in temper T4, as delivered, particularly for stamping operations, - Controlled tensile yield strength in the as-delivered condition of the sheet to reduce springback during shaping; - Excellent behavior in the various assembly methods used in automotive bodies, such as spot welding, laser welding, adhesive bonding, clinching or riveting, - High mechanical strength after electrophoresis and paint baking to minimize the weight of the part while obtaining excellent mechanical strength in operation, - Excellent energy absorption capacity during impact when applied to vehicle body structural parts; - Excellent resistance to corrosion, in particular intergranular, stress and filiform corrosion of the finished part; - Compliance with requirements for recycling of manufacturing waste or recycled vehicles; - Acceptable mass production costs.

[0007] However, there are currently mass-produced motor vehicles with bodies-in-white constructed primarily from aluminum alloys. For example, the 2014 version of the Ford F-150 model is constructed from AA6111 structural alloy. This alloy was developed by the Alcan Group in the 1980s and 1990s. The following two references describe this development work: - P.E. Fortin et al., "An optimized Al alloy for auto body sheet applications", SAE technical conference, March 1984, describes a composition of Si: 0.85; Fe: 0.20; Cu: 0.75; Mn: 0.20 and Mg: 0.72. - M.J. Bull et al., "Al sheet alloys for structural and skin applications," 25th ISATA symposium, Paper 920669, June 1992.

[0008] Although originally designed to withstand press-fitting for skin-type applications, "a yield strength of 280 MPa is achieved after 2% pre-strain and 30 minutes at 177°C," its primary property continues to be high mechanical strength.

[0009] Furthermore, other alloys within the AA6xxx family have been developed with high mechanical properties for aeronautical or automotive applications. Thus, the AA6056 type alloys, whose development dates back to the 1980s at Pechiney, have been the subject of much research work and numerous publications in order to optimize their mechanical properties or improve their resistance to intergranular corrosion. This was the subject of one patent application (WO 2004 / 113579).

[0010] AA6013 type alloys have also been the subject of much research work. For example, Alcoa, in U.S. Patent Application Publication No. 2002 / 039664, published in 2002, reported that an alloy containing 0.6-1.15% Si; 0.6-1% Cu; 0.8-1.2% Mg; 0.55-0.86% Zn; less than 0.1% Mn; 0.2-0.3% Cr and approximately 0.2% Fe, used in the T6 temper, had excellent resistance to intergranular corrosion and a Rp of 380 MPa. 0.2 It is a combination of the following.

[0011] Aleris, WO 2003 / 006697, published in 2003, relates to alloys within the AA6xxx series with 0.2% to 0.45% Cu. The object of the invention is to propose an AA6013-type alloy with reduced Cu levels, targeting an Rm of 355 MPa in the T6 temper and excellent resistance to intergranular corrosion. The claimed composition is as follows: 0.8-1.3% Si, 0.2-0.45% Cu; 0.5-1.1% Mn; 0.45-0.1% Mg.

[0012] Structural parts for automotive applications made from 7xxx alloys, as described, for example, in EP 2581218, are also known.

[0013] Furthermore, in order to produce parts from aluminum alloys with complex geometries, such as door linings, which cannot be achieved by conventional stamping using the above-mentioned alloys, various solutions have been contemplated and / or implemented in the past, such as: - Avoiding the difficulties associated with stamping by producing parts of this type by molding, in particular by molding of the "under pressure" type. European Patent No. 1305179 of Nothelfer GmbH, with priority dated 2000, attests to this. - To benefit from better formability, so-called "warm" stamping is carried out, which consists in heating the aluminum alloy blank globally or locally to so-called intermediate temperatures, i.e., 150-350°C, to improve its behavior under presses with tools that can also be preheated. The applicant's EP 1 601 478, with priority from 2003, is based on this solution. - Modifying the stamping suitability of the alloys within the AA5xxx series through their composition, in particular by increasing the magnesium content above 5%, which is not neutral from the point of view of corrosion resistance. - Use composite sheets consisting of a core alloy from the AA5xxx series with more than 5% Mg for better formability and clad sheets made from alloys with better corrosion resistance. However, the corrosion resistance at the edges of the sheets, in the perforation zones or more generally where the core is exposed, and especially within the assembly, may then prove insufficient. Furthermore, EP 1702995 A1 discloses a method for producing an aluminum alloy sheet, which comprises molten aluminum alloy having a chemical composition, in weight percent, of 0.30-1.00% Mg, 0.30-1.20% Si, 0.05-0.50% Fe, 0.05-0.50% Mn, 0.005-0.10% Ti, optionally one or more of 0.05-0.70% Cu and 0.05-0.40% Zr, with the remainder being Al and unavoidable impurities. The document describes a method comprising the steps of: providing; casting the molten alloy into a plate having a thickness of 5-15 mm by a double strip casting method at a cooling rate of 40-150°C / s at one-quarter of the thickness of the plate; coiling into the form of a reel; homogenization; cooling the resulting reel to a temperature of 250°C at a cooling rate of at least 500°C / h, followed by cold rolling and then a solution heat treatment. The document does not mention ageing on the part after forming. - WO 2018 / 185425 discloses a method for producing stamped components of a body or body structure of a motor vehicle from an aluminum alloy containing (in weight percent) Si: 0.60-0.85; Fe: 0.05-0.25; Cu: 0.05-0.30; Mn: 0.05-0.30; Mg: 0.50-1.00; Ti+V≦0.10, where Ti: 0.02-0.10; V: 0.00-0.10; other elements each less than 0.05 and total less than 0.15, the remainder being aluminum, where Mg<-2. The invention relates to a method for producing a metal sheet or strip having a thickness of 1.0 to 3.5 mm and having an alloy of composition 67 × Si + 2.87, comprising the steps of melting and dipping, pre-tempering, aging for 72 hours to 6 months, stamping, tempering at a temperature of around 205°C with a holding time of 30 to 170 minutes or an equivalent time-temperature tempering step, painting, and "baking" the paint at a temperature of 150 to 190°C for 15 to 30 minutes. The invention also relates to a stamped component of a motor vehicle body or body structure, also known as a "body in white," produced by such a method.

[0014] U.S. Patent Application Publication No. 2018 / 0119261 describes 6xxx series aluminum alloys with unexpected properties and novel methods for producing such aluminum alloys. The aluminum alloys are highly formable and exhibit high strength. The alloys can be produced by continuous casting and hot rolled to final gauge and / or temper. The alloys can be used in automotive, transportation, industrial, and electronics applications, to name a few.

[0015] U.S. Patent Application Publication No. 2018 / 0171452 discloses high-strength, highly deformable aluminum alloys and methods for manufacturing and processing such alloys. More specifically, heat-treatable aluminum alloys are disclosed that exhibit improved mechanical strength and formability. Processing methods include casting, homogenizing, hot rolling, solutionizing, preaging, and in some cases, prestraining. In some cases, processing steps can further include cold rolling and / or heat treating.

[0016] Considering the increasing developments in the use of aluminum sheet for automotive body components and mass production, there is still a demand for ever improved grades that allow thickness reduction without compromising other properties, always with the aim of improving weight reduction. [Prior art documents] [Patent documents]

[0017] [Patent Document 1] International Publication No. 2004 / 113579 [Patent Document 2] US Patent Application Publication No. 2002 / 039664 [Patent Document 3] International Publication No. 2003 / 006697 [Patent Document 4] European Patent No. 2581218 [Patent Document 5] European Patent No. 1305179 [Patent Document 6] European Patent No. 1601478 [Patent Document 7] European Patent Application Publication No. 1702995 [Patent Document 8] International Publication No. 2018 / 185425 [Patent Document 9] US Patent Application Publication No. 2018 / 0119261 [Patent Document 10] US Patent Application Publication No. 2018 / 0171452 [Non-patent literature]

[0018] [Non-Patent Document 1] P.E. Fortin et al., "An optimized Al alloy for auto body sheet applications," SAE technical conference, March 1984 [Non-patent document 2] M.J. Bull et al., "Al sheet alloys for structural and skin applications," 25th ISATA symposium, Paper 920669, June 1992 Summary of the Invention [Problem to be solved by the invention]

[0019] The present invention aims to obtain an excellent compromise between good riveting and crash behavior of the finished component, as well as high mechanical strength and formability in the T4 temper, by proposing a manufacturing method for components including a forming step in the T4 temper after natural ageing at ambient temperature, optionally followed by an age-hardening step on the formed part and a paint baking or bake-hardening step. One challenge is also to achieve a method that is fast and economically advantageous, and which represents an improvement over products made from AA6111 alloy.

[0020] These components must also have excellent behavior in various assembly processes such as spot welding, laser welding, adhesive bonding, clinching or riveting, and very good corrosion resistance. [Means for solving the problem]

[0021] The subject of the present invention is a method for producing an automotive body or a body structural rolled product, also called a "body in white", from an aluminum alloy, comprising the steps of: a.Si:0.75~1.10; Fe: max 0.4; Cu: 0.5~0.8; Mn: 0.1~0.4; Mg: 0.75~1; Ti: max 0.15; Cr: max 0.1; V: max 0.1; Unavoidable elements and impurities: each up to 0.05%, total up to 0.15%; The rest is aluminum; Casting an ingot having a composition (wt%) of: b. Ingot homogenization step; c. Hot rolling of the ingot; d. Cold rolling into sheet; e. Solution heat treatment and quenching steps for the sheet; f. Sheet pre-aging step; g.Natural aging step of the sheet; The method includes the following successive steps:

[0022] Another subject of the invention is a rolled product obtainable by the method of the invention.

[0023] Another subject of the invention is a part obtainable by the method of the invention.

[0024] Another subject of the invention is the use of the part in a motor vehicle as body skin parts (or outer panels of a vehicle body), such as front wings, roof, bonnet, trunk or door skins, and lining parts or body structural components, such as doors, bonnet, tailgate or roof linings or reinforcements, or spars, bulkheads, load floors, floor tunnels and front, middle and rear pillars, and finally impact absorbers or "crash boxes". [Brief explanation of the drawings]

[0025] [Figure 1] The figure shows a "three-point bending test" apparatus consisting of two rollers R and a punch B of radius r for bending a rolled product T of thickness t. [Figure 2] The rolled product T is depicted after a "three-point bending" test with an interior angle β, and the exterior angle α measured during the test is reported in the accompanying results, as is the maximum strength during the test procedure. [Figure 3] A particular embodiment of the method is depicted. DETAILED DESCRIPTION OF THE INVENTION

[0026] Unless otherwise defined herein, general terms are as defined in the NF EN12258-1 standard. Sheet is a flat rolled product with a rectangular cross section having a uniform thickness of 0.20 mm to 6 mm.

[0027] Unless otherwise indicated, all aluminum alloys under consideration below are designated by the designations defined by the Aluminum Association in its Registration Record Series, published periodically by the Association.

[0028] All designations regarding the chemical composition of the alloy are expressed as weight percent based on the total weight of the alloy.

[0029] Definitions of metallurgical tempers are given in European Standard EN515 unless otherwise defined herein.

[0030] Static tensile mechanical properties, i.e., ultimate tensile strength Rm, tensile yield strength Rp at 0.2% elongation 0.2 , and the elongation at break A % are determined by tensile testing according to NF EN ISO 6892-1.

[0031] The bending angle is determined by a three-point bending test according to NF EN ISO 7438 and procedures VDA 238-100 and VDA 239-200.

[0032] Flexibility is also measured using ASTM standard E290-97a.

[0033] The inventors have selected a set of aluminum alloy compositions along with a preferred method that provides automakers with advantageous properties for producing parts.

[0034] The subject of the present invention is a method for producing an automotive body or a body structural rolled product, also called a "body in white", from an aluminum alloy, comprising the following steps: Casting an ingot having the following composition (in weight percent): Si: 0.75-1.10. Preferably, the maximum Si content is 1.0%, and more preferably, the maximum Si content is 0.95%. Fe: max 0.4. Preferably, the minimum Fe content is 0.15% and / or the maximum Fe content is 0.30%. Cu: 0.5 to 0.8. Preferably, the maximum Cu content of the ingot is 0.70% and / or the minimum Cu content is 0.55%. More preferably, the maximum Cu content is 0.65%. Because Cu is typically more expensive than aluminum, limiting Cu to 0.8%, 0.70%, or even 0.65% is advantageous for economic reasons. It is also advantageous for facilitating material recycling. It can also improve corrosion resistance. However, in another embodiment, the minimum Cu content is 0.65% to particularly increase strength. Mn: 0.1-0.4. Preferably, the maximum Mn content is 0.35% and / or the minimum Mn content is 0.24% or preferably 0.25%. Addition of Mn improves, in particular, the bending behavior. Mg: 0.75-1, preferably the minimum Mg content is 0.80% and / or the maximum Mg content is 0.90%. Ti: max 0.15, preferably the minimum Ti content is 0.01% and / or the maximum Ti content is 0.05%. Cr: max 0.1, and preferably Cr is an unavoidable element or impurity. V: maximum 0.1, and preferably V is an unavoidable element or impurity. and unavoidable elements and impurities of up to 0.05% each, totaling 0.15%, with the remainder being aluminum.

[0035] Casting can be carried out using various casting processes. Continuous casting, usually horizontal casting, is possible. It is also preferable to use vertical semi-continuous casting, also known as direct chill casting. Vertical semi-continuous casting is preferred because it is more homogeneous throughout the thickness of the sheet.

[0036] The ingot is homogenized, hot rolled, and cold rolled to form a sheet. The sheet is solution heat treated and quenched. Preferably, the ingot is homogenized at a temperature of 520-560°C, preferably for 2-8 hours. Preferably, hot rolling rolls the ingot into an intermediate rolled product having a thickness of 3-10 mm. Preferably, cold rolling rolls the intermediate rolled product into a sheet having a thickness of 1-4 mm. The sheet is then solution heat treated, typically at a temperature above the solvus temperature of the alloy, while avoiding incipient melting. Preferably, the solution heat treatment temperature is from 530°C, preferably from 540°C to 580°C, for a time preferably of 1 second to 5 minutes. The sheet is then quenched. Water quenching at a temperature of about 15-60°C, preferably from 15°C to 40°C, is suitable. Pre-aging is preferably applied at a temperature of 50 to 120°C, preferably for at least 8 hours. Natural aging is then applied, as defined in NF EN 12258-1, and room temperature as defined in NF EN ISO 6892-1. Preferably, the duration of natural aging is 72 hours to 6 months.

[0037] The pre-aging step is preferably accomplished by coiling the sheet at the coiling temperature and allowing it to cool at room temperature in open air.

[0038] A convenient continuous annealing line for achieving pre-aging is depicted in FIG. 3. The sheet 3 is uncoiled by an uncoiler 1, passes through a solution furnace 4 and a quenching unit 5, then enters a surface treatment unit 6, a step very useful for body sheet, followed by a pre-aging oven 7, and finally coiled on a coiler 2 in open air. Thus, at the exit of the pre-aging oven 7, the sheet is hot and is coiled on the coiler 2 in open air at the coiling temperature. The coiled sheet 8, still hot, is stored at ambient temperature in the plant and allowed to cool to ambient temperature. Pre-aging occurs during this cooling period. Natural aging begins after the coiled sheet 8 has cooled, and the pre-aging duration is preferably at least 8 hours.

[0039] Preferably, the pre-aging is obtained by coiling the sheet at a coiling temperature of 50 to 120°C, preferably 60 to 120°C, followed by cooling the coiled sheet in open air, the duration of which is at least 8 hours.

[0040] The rolled product of the present invention includes products that can be obtained by the above-mentioned method from casting to natural aging. The temper of the rolled product after natural aging is T4.

[0041] The tensile yield strength of rolled products in the T4 temper varies within the same roll between the tensile yield strength in the transverse and 45° directions by less than 5 MPa, preferably less than 3 MPa. Identical sheet is defined as rolled products made from the same ingot, homogenized, hot and cold rolled, solution heat treated, quenched, pre-aged, and naturally aged, and tensile test specimens are cut from the rolled products as close as possible. This is a useful property for part stamping.

[0042] Products rolled in the T4 temper can be characterized in six other specific tempers, T8A, T8C, T8D, T6B, T6C and T8D, which estimate the material properties of the part.

[0043] The T8A, T8C, and T8D tempers are achieved by applying 2% strain to T4 rolled product followed by a specific heat treatment. The T8A temper uses a 20-minute bake-hardening heat treatment at 180°C. The T8C temper uses a 5-minute light, short bake-hardening heat treatment at 160°C. The T8D temper uses a 20-minute light, long bake-hardening heat treatment at 160°C.

[0044] The T6B, T6C, and T6D tempers are achieved by applying specific heat treatments to the T4 rolled product. The T6B temper uses a heat treatment at 225°C for 30 minutes. The T6C temper uses a light, short bake hardening heat treatment at 160°C for 5 minutes. The T6D temper uses a light, long bake hardening heat treatment at 160°C for 20 minutes.

[0045] The T4 rolled product can then be formed, in particular by press stamping, to obtain a shaped product. Optionally, the shaped product is aged. The shaped product can be painted and bake-hardened to a part at a temperature of 150-190°C, preferably 170-190°C, for 5-30 minutes, preferably 15-30 minutes.

[0046] The subject of the present invention are parts obtainable by the above-mentioned method using the rolled product of the present invention, which parts can be used in motor vehicles as body skin parts (or outer panels of the body), such as front wings, roofs, bonnets, trunks or door skins, and lining parts or body structural components, such as doors, bonnets, tailgates or roof linings or reinforcements, or preferably spars, bulkheads, load floors, floor tunnels and front, middle and rear pillars, and finally impact absorbers or "crash boxes".

[0047] In a first embodiment, the coiling temperature is from 50°C to 95°C, excluding 95°C, and preferably from 60°C to 95°C, excluding 95°C. The rolled product in the T4 temper of this first embodiment is characterized by a tensile yield strength of less than 165 MPa, which may be useful for customer formability in press stamping. The rolled product in the T6B temper of this first embodiment has a minimum tensile yield strength of 345 MPa, and preferably a minimum tensile yield strength of 350 MPa, as formally described.

[0048] A preferred composition for the method according to the first embodiment is as follows: Si: 0.75 to 1.10, preferably less than 0.95%; Fe: maximum 0.4, more preferably 0.15%-0.30%; Cu: 0.5-0.70, preferably 0.5-0.65; Mn: 0.1~0.4; Mg: 0.75~1; Ti: 0.01~0.05; Cr: max 0.1; V: as an impurity; Incidental elements and impurities: maximum 0.05% each, maximum 0.15% in total; The remainder is aluminum.

[0049] With this preferred composition, and at coiling temperatures of 50°C to 95°C, excluding 95°C, and preferably 60°C to 95°C, excluding 95°C, the bendability of the T4 rolled product of the first embodiment is up to 0.19, which is advantageous for part forming.

[0050] An even more preferred composition of the first embodiment is as follows: Si: 0.75 to 1.10, preferably less than 0.95%; Fe: maximum 0.4, more preferably 0.15%-0.30%; Cu: 0.5-0.70, preferably 0.5-0.65; Mn: 0.24-0.30, preferably minimum 0.25%; Mg: 0.75~1; Ti: 0.01~0.05; Cr: max 0.1; V: as an impurity; Incidental elements and impurities: maximum 0.05% each, maximum 0.15% in total; The remainder is aluminum.

[0051] In this even more preferred composition, in conjunction with a coiling temperature of 50° to 70° C., preferably 60° to 70° C., the VDA angle of the rolled product in the T4 temper is greater than 125°. The bendability of the T4 rolled product is still less than 0.19, which can be advantageous in some press stamping applications.

[0052] In another preferred method of the first embodiment, the coiling temperature is 70°C to 95°C. In this method, the rolled product in T8A temper has a minimum tensile yield strength of 275 MPa. In a more preferred method of this embodiment, the rolled product in T8A temper has a minimum tensile yield strength of 280 MPa at a coiling temperature of 70°C to 95°C and with the following composition: Si: 0.75 to 1.10, preferably less than 0.90%; Fe: maximum 0.4, more preferably 0.15%-0.30%; Cu: 0.65~0.8; Mn: 0.1-0.4, more preferably less than 0.24%, minimum 0.15%; Mg: 0.75 to 1, preferably less than 0.95%; Ti: 0.01~0.05; Cr: max 0.1; V: as an impurity; Incidental elements and impurities: maximum 0.05% each, maximum 0.15% in total; The remainder is aluminum.

[0053] In the second embodiment of the present invention, the coiling temperature is 95°C to 120°C, preferably 95°C to 105°C, and the composition is preferably as follows: Si: 0.75 to 1.10, preferably less than 0.90%; Fe: maximum 0.4, more preferably 0.15%-0.30%; Cu: 0.5-0.70, preferably 0.5-0.65; Mn: 0.1-0.4, preferably minimum 0.25% and preferably less than 0.35%; Mg: 0.75~1; Ti: 0.01~0.05; Cr: max 0.1; V: as an impurity; Incidental elements and impurities: maximum 0.05% each, maximum 0.15% in total; The remainder is aluminum.

[0054] The advantage of this second embodiment is specifically that the yield strength of the part is less sensitive to variations in the bake hardening process. Bake hardening conditions are dependent on the location within the body assembly, and therefore parts that are less sensitive to bake hardening conditions are advantageous because they offer greater flexibility to the automaker. This sensitivity can be assessed by comparing the properties in T6C and T6D tempers and / or T8C and T8D tempers from the same T4 temper rolled product.

[0055] In the rolled products obtained by the method of the second embodiment, the difference in tensile yield strength between the rolled products in the T8C and T8D tempers and the same rolled products in the T4 temper is less than 5 MPa. The T8C and T8D rolled product samples differ only in the duration of bake hardening, which is 160°C.

[0056] The only difference between the T6C and T6D rolled product samples is the duration of bake hardening, which is 160°C. In the rolled products obtained by the method of the second embodiment, the difference in tensile yield strength between the rolled products in the T6C and T6D tempers and the same rolled products in the T4 temper is less than 5 MPa.

[0057] More generally, the rolled product may be heat treated at a temperature of 150-190°C, preferably 170-190°C, for a period of 5-30 minutes, preferably 15-30 minutes. The yield strength of a rolled product heat treated at a given temperature within the above temperature range and for any duration within the above duration range will vary by less than 15 MPa, preferably less than 10 MPa and more preferably less than 5 MPa.

[0058] More generally, the 2% strained rolled product may be heat treated at a temperature of 150-190°C, preferably 170-190°C, for a time period of 5-30 minutes, preferably 15-30 minutes. The yield strength of a 2% strained rolled product heat treated at a given temperature within the above temperature range and for any duration within the above duration range will vary by less than 15 MPa, preferably less than 10 MPa and more preferably less than 5 MPa.

[0059] In a second embodiment, the rolled product in a T4 temper has a maximum tensile yield strength of 190 MPa. In a second embodiment, the rolled product in a T6B temper has a minimum tensile yield strength of 340 MPa. In a second embodiment, the rolled product in a T8A temper has a minimum tensile yield strength of 280 MPa, preferably 290 MPa.

[0060] The recyclability of any alloy is an important technical and economic parameter. Reducing the range of any element is useful for enhancing the recycling process, since it provides predictability of future melts. Reducing the maximum values ​​of additional elements is also advantageous, since these elements may be more expensive than aluminum. Reducing the Si content is advantageous for recycling, since this element is not only an impurity in many alloys but also detrimental to the properties of aluminum products. Therefore, an advantageous embodiment of the present invention consists in reducing the Si content to a maximum of 0.95%. It is also advantageous to reduce the Fe maximum value to 0.30% and / or increase the Fe minimum value to 0.15%. Another advantageous embodiment consists in reducing the Cu maximum value to 0.70%, preferably 0.65%, and / or increase the Cu minimum value to 0.55%. Another advantageous embodiment consists in reducing the Mn maximum content to 0.35%, more preferably 0.30%, and / or increase its minimum content to 0.15%, more preferably 0.25%. Another embodiment consists in reducing the maximum content of Ti to 0.05% and / or increasing the minimum content to 0.01%. Another embodiment consists in classifying V as an impurity with a maximum content of 0.05%.

[0061] All of these combinations of alloy compositions and coiling temperatures of the present invention offer many possibilities to automakers with different forming properties. Automakers can also optimize their processing and their part designs. Shape aging allows for high strength parts, but requires specific heat treatments for shape aging. High strength alloys are useful for lightweight parts. If the part does not require high strength material, automakers can avoid shape aging, which is advantageous for simplifying production. Thus, the present invention offers flexibility to automakers. [Example]

[0062] preface Table 1 summarizes the chemical composition (wt%) of the alloys used during the tests. The percentage of other unavoidable elements and impurities is less than 0.05%, and the total is less than 0.15%, with the remainder being aluminum. Alloy G is an exemplary AA6111 alloy, and Alloy H is an example of a modified AA6056.

[0063] [Table 1]

[0064] Rolled ingots of these various alloys were obtained by vertical semi-continuous casting. After scalping, these various ingots were subjected to homogenization heat treatment at 540°C for about 4 hours and immediately hot-rolled to obtain 5 mm intermediate rolled products. These 5 mm intermediate rolled products were then cold-rolled to obtain 2 mm thick sheets.

[0065] The rolling step was followed by a solution heat treatment followed by quenching. The solution heat treatment was carried out above the solvus temperature of the alloy while avoiding incipient melting. In this non-limiting example, the solution temperature was 570°C. The solutionized sheet was then water quenched in water at 20°C. The sheet samples were coiled at three coiling temperatures: 100°C, 80°C, and 60°C for an 8-hour pre-aging period, followed by natural aging. Two natural aging periods, namely 7 and 30 days at room temperature, were used to obtain rolled products in T4 temper.

[0066] The T4 mill was converted to a T8A temper using a 2% strain followed by a heat treatment with a typical bake hardening at 180° C. for 20 minutes. The T8A specimen was then characterized.

[0067] The T4 mill was also heat treated to a T6B temper using a 30 minute 225°C heat treatment. The T6B samples were then characterized.

[0068] Test results Tensile tests were performed at ambient temperature using non-proportional specimens with a geometry commonly used for sheet, corresponding to specimen type 2 in Table B.1 of Appendix B of this standard, in accordance with NF EN ISO 6892-1. These specimens specifically had a width of 20 mm and a calibrated length of 120 mm. Tensile tests were performed on rolled products in T4, T8A, and T6B tempers. Results obtained at a coiling temperature of 80°C and 30 days of natural aging are presented in Table 2. Results obtained at a coiling temperature of 60°C and 30 days of natural aging are presented in Table 3. Results obtained at coiling temperatures of 60°C, 80°C, and 100°C and 7 days of natural aging are presented in Table 4.

[0069] [Table 2]

[0070] [Table 3]

[0071] [Table 4]

[0072] The coiling temperature is an important parameter for determining the tensile yield strength of the T4 temper. At 60°C and 80°C, it is possible to limit the T4 tensile yield strength to less than 165 MPa, which can be advantageous for automakers if they need to maintain ease of stamping.

[0073] Example alloys B, D, E and F have a minimum tensile yield strength of 350 MPa in the T6B temper. These example alloys have a minimum tensile yield strength of 275 MPa in the T8A temper.

[0074] Reducing the range of Ti to a maximum of 0.05%, V to a maximum of 0.05% impurity, and Cu to less than 0.65% is also advantageous, as demonstrated by alloys E and D, as it reduces the bendability to 0.15, making the component easily manufacturable independent of the coiling temperature.

[0075] Optimizing the Mn range from 0.25 to 0.35% in addition to the reduced V, Ti, and Cu ranges mentioned above provides very favorable three-point bend test results at a coiling temperature of 60°C along with a high VDA angle which is good for formability. This is demonstrated by Alloy E at a coiling temperature of 60°C.

[0076] Example 2 After natural aging for 7 days at coiling temperatures of 80°C and 100°C, rolled products made of Alloy E were used for further tests. The specimens at both coiling temperatures were divided into two groups: the first group was subjected to a 2% strain and the second group was free of any strain. Then, a bake hardening temperature of 160°C was applied for two different durations: 5 minutes and 20 minutes.

[0077] These results, provided in Table 5 for a coiling temperature of 80°C and in Table 6 for a coiling temperature of 100°C, demonstrate another advantageous embodiment: at a coiling temperature of 100°C, the tensile yield strength of the rolled product is nearly independent of the duration of bake hardening. This is advantageous behavior for parts that can be installed either on the surface of a multi-component assembly or deep within it within a body assembly, since the yield strength remains similar. This provides automotive manufacturers with flexibility in part design.

[0078] [Table 5]

[0079] [Table 6]

[0080] Example 3 Ingots of the following compositions were cast:

[0081] Ingots having the chemical composition (wt%) in Table 7 were cast using vertical semi-continuous casting. The proportion of other unavoidable elements and impurities was less than 0.05%, with the total being less than 0.15%, the remainder being aluminum.

[0082] [Table 7]

[0083] The rolled ingot was heated to 554°C for 4 hours. The ingot was directly hot rolled. The ingot temperature just before the start of hot rolling was 540°C. At the end of hot rolling, the thickness was 5 mm. At the end of cold rolling, the thickness was 2 mm. The sheet was divided into three sections for solution treatment at three different temperatures: 20 seconds, 45 seconds, and 68 seconds above 525°C, and 535°C and 544°C. The sheet was quenched in water at 22°C. The sheet was pre-aged by coiling at 96°C, cooling in open air, and then natural aging at room temperature of approximately 20°C for 3 days to obtain a rolled product with a T4 temper.

[0084] The T4 mill was converted to a T8A temper using a 2% strain followed by a heat treatment with a typical bake hardening at 180° C. for 20 minutes. The T8A specimen was then characterized.

[0085] The T4 mill was also heat treated to a T6B temper using a 30 minute 225°C heat treatment. The T6B samples were then characterized.

[0086] Tensile tests were performed in the rolling direction (L), transverse to the rolling direction (T) and at 45° to the rolling direction (45°).

[0087] [Table 8]

[0088] Table 8 shows that the solution heat treatment is highly reliable against process variations in temperature or duration to obtain mechanical properties.

[0089] The tensile yield strength of the T4 temper shows the same anisotropy of less than 3 MPa between the tensile yield strength in the T direction and the 45° direction within the rolled product as seen in Table 8.

[0090] To check the impact behavior of the rolled products, the bending radius was also measured for the T6B temper. The results are shown in Table 9.

[0091] [Table 9] [Explanation of symbols]

[0092] 1: Uncoiler 2: Koira 3: Sheet 4: Solution furnace 5: Quenching unit 6: Surface treatment machine 7: Pre-aging oven 8: Stored coils

Claims

1. 1. A method for producing an automotive body or body structural rolled product, also known as a "body in white," from an aluminum alloy, comprising: a. Si of 0.75 to 1.10, Fe up to 0.4, 0.5 to 0.8 Cu, Mn from 0.1 to 0.4, 0.75 to 1 Mg, Ti up to 0.15, Cr up to 0.1, V up to 0.1, Incidental elements and impurities up to 0.05% each and up to 0.15% in total; The rest is aluminum, Casting an ingot having a composition (by weight) of b. A homogenization step of the ingot at a temperature of 520-560°C; c. hot rolling the ingot to a thickness of 3-10 mm; d. cold rolling into a sheet to a thickness of 1-4 mm; e. Solution heat treatment and quenching of the sheet at a temperature of 540-580°C; f. Pre-aging the sheet at a temperature of 50°C to 120°C for at least 8 hours by coiling the sheet at a coiling temperature of 50°C to 120°C; g. Natural aging step at ambient temperature of the sheet to T4; A method comprising the following successive steps:

2. 2. The method of claim 1, wherein the maximum Cu content of the ingot is 0.70%.

3. 3. The method according to claim 1 or 2, characterized in that the maximum Mn content of the ingot is 0.35%.

4. 4. A method according to any one of claims 1 to 3, characterized in that the maximum Ti content of the ingot is 0.05%.

5. A method according to any one of claims 1 to 4, characterized in that the maximum V content of the ingot is 0.05%.

6. The method of claim 1, wherein the casting step is a vertical semi-continuous casting step.

7. 2. The method of claim 1, wherein the pre-aging is achieved by coiling the sheet at a coiling temperature between 70°C and 95°C, excluding 95°C.

8. 2. The method of claim 1, wherein the pre-aging is achieved by coiling the sheet at a coiling temperature between 50°C and 70°C.

9. 2. The method of claim 1, wherein the pre-aging is achieved by coiling the sheet at a coiling temperature above 95°C.

10. The tensile yield strength of the rolled product in the T4 temper is less than 165 MPa, and the tensile yield strength of the rolled product in the T6B temper is at least 345 MPa; 0.75% to 1.10% Si, max. 0.4% Fe, 0.5% to 0.70% Cu, 0.1% to 0.4% Mn, 0.75% to 1% Mg, 0.01% to 0.05% Ti, 0.1% max Cr, Maximum 0.05% V as an impurity Incidental elements and impurities up to 0.05% each and up to 0.15% in total; The rest is aluminum, Having the composition Rolled products.

11. A rolled product as described in claim 10, wherein the bendability of the T4 rolled product is up to 0.

19.

12. A rolled product according to claim 10, wherein Mn is 0.24% to 0.30%.

13. The tensile yield strength of the rolled product in the T8A temper is at least 275 MPa; 0.75% to 1.10% Si, max. 0.4% Fe, 0.65% to 0.8% Cu, less than 0.24% and a minimum of 0.15% Mn; 0.75% to 1% Mg, 0.01% to 0.05% Ti, Maximum 0.05% V as an impurity Incidental elements and impurities up to 0.05% each and up to 0.15% in total; The rest is aluminum, Having the composition Rolled products.

14. the difference in tensile yield strength between the rolled products in T8C and T8D tempers and the same rolled product in T4 temper is less than 5 MPa, and the difference in tensile yield strength between the rolled products in T6C and T6D tempers and the same rolled product in T4 temper is less than 5 MPa; 0.75% to 1.10% Si, max. 0.4% Fe, 0.5% to 0.70% Cu, 0.1% to 0.4% Mn, 0.75% to 1% Mg, 0.01% to 0.05% Ti, 0.1% max Cr, Maximum 0.05% V as an impurity Incidental elements and impurities up to 0.05% each and up to 0.15% in total; The rest is aluminum, Having the composition Rolled products.

15. h. Forming the rolled product into a shaped product; i. optionally, an artificial ageing step of said shape; j. Painting and "bake-curing" the shapes into one piece at a temperature of 150-190°C for 5-30 minutes; 10. The method of claim 1, further comprising the following successive steps:

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