EP3765647B1

EP3765647B1 - Method of manufacturing an almgsi alloy sheet product

Info

Publication number: EP3765647B1
Application number: EP19705361.4A
Authority: EP
Inventors: Mehdi LALPOOR
Original assignee: Aleris Aluminum Duffell BVBA
Current assignee: Aleris Aluminum Duffell BVBA
Priority date: 2018-03-15
Filing date: 2019-02-19
Publication date: 2023-05-31
Anticipated expiration: 2039-02-19
Also published as: EP3765647A1; WO2019174870A1

Description

FIELD OF THE INVENTION

The invention relates to a method of manufacturing an AA6000-series aluminium alloy sheet product, which is particularly suitable for use in the production of automotive body parts, and to an aluminium alloy sheet product of the AA6000-series, produced by this method.

BACKGROUND TO THE INVENTION

The increasing stipulations of lower CO₂ emissions in the automotive industry have led to an increased use of aluminium alloys in the manufacture of automotive vehicles, because of their lower weight as compared to steel. The AA6000-series (AlMgSi and AlMgSiCu) sheet alloys have been used in the production of automotive outer panels and other body parts, because of their excellent combination of good formability and hemming performance in the T4 or T4P temper, combined with a significant bake-hardening response after paint-baking. In order to enhance the paint-bake response, it is known to apply a pre-aging treatment to the sheet product before shipping it to the automobile manufacturer.
It has been found that the paint-bake response is affected by the natural aging time prior to the paint-bake cycle. A considerable natural ageing time, however, is inevitable in regular industrial production, as the sheet material is stored and transported from the rolling mill to the automobile manufacturer, where it is usually stored for a considerable time prior to being further processed into an automotive body part. To eliminate the negative effect of natural aging on paint bake response, it is known to apply a pre-aging treatment to the sheet product before shipping it to the automobile manufacturer.
US 2016/0348225 A1 discloses a method of preparing a AA6000-series aluminium alloy sheet product, which includes solution heat treating and then quenching the AA6000-series aluminium alloy sheet product, followed by exposure of the sheet product to a temperature range of 30-60°C for 0.2-300 seconds. Afterwards, the sheet product is coiled and then placed in an ambient environment. By this method, the AA6000-series aluminium alloy sheet product may realise more consistent strength and ductility.
EP 0 805 879 B1 discloses a process of producing an aluminium alloy sheet material, which comprises subjecting hot-or cold-rolled aluminium alloy sheets to solution heat treatment followed by quenching and, before substantial age-hardening has taken place, subjecting the alloy sheet material to one or more subsequent heat treatments involving heating the material to a peak temperature in the range of 100-300°C, preferably 130-270°C, holding the material at the peak temperature for a period of time less than about 1min, and cooling the alloy from the peak temperature to a temperature of 85°C or less.
US 2016/0222491 A1 discloses an AA6000-series aluminium alloy sheet product with good bendability, which is produced by solution heat treatment, followed by quenching. After quenching, the sheet is subjected to pre-aging within 1hr. The pre-aging treatment consists of holding the sheet at 60-120°C for ten hours to 40 hours.
CN 107254646 discloses a heat treatment method for 6000 aluminium series alloys including solution heat treatment and quenching to 40-60°C, soaking for 5-20 min, and then rapidly heating to 100-120°C, and holding at this temperature for 2-5 min.
EP 2 110 235 A1 discloses an aluminium alloy sheet product with improved hemming performance suitable for use for an automotive body and a method of forming an aluminum alloy sheet product comprising casting a body of an aluminium alloy, working said body to produce a sheet, solution heat treating said sheet, and pre-ageing and naturally ageing said sheet.
WO 96/38598 discloses a method of producing an aluminium article comprising the steps of providing a stock including an aluminium alloy, hot working the stock, solution heat treating at a temperature from 900 to 1100 °F for 2.0 seconds to about 30 minutes, rapid quenching, cooling to room temperature, holding at room temperature for not more than 24 hours and reheating to a temperature ranging from 150 to 360 °F for 24 hours to 2.0 minutes.
US 2008/0178973 A1 discloses a method for producing an aluminium alloy sheet with excellent bendability and paint bake hardenability, comprising homogenizing an ingot at 450 °C or more, cooling the ingot to a temperature less than 350 °C, hot-rolling the ingot, cold-rolling the hot-rolled product and a solution heat treatment at 450 °C and more, and quenching.
CN105441740A discloses a method for manufacturing an aluminium alloy sheet comprising melting the alloy, homogenizing the ingot at 500-580 °C, cooling to a hot-rolling temperature, hot-rolling the ingot, cold rolling and annealing to obtain a cold sheet, solution heat treatment at 500-560 °C and pre-aging at 60-100 °C.
JP2007031819A discloses an aluminum alloy with excellent bendability and rough-skin prevention property by performing sequentially a first cold-rolling and middle solution formation, a second cold-rolling and last solution formation and a preliminary backup aging, and performing said second cold rolling by 15 to 30 % of the last cold-rolled rate.

OBJECT OF THE INVENTION

The pre-aging treatments known from the prior art mostly have the objective of stabilizing the paint-bake response, i.e. the increase in yield strength induced by a paint bake cycle. However, often the prior art has focused merely on the paint-bake response, without paying attention also to the bending behaviour and related hemming performance of the pre-aged sheet product.
It is therefore an object of the invention to provide a method for manufacturing an 6000-series aluminium alloy sheet product ideally for use in the production of automotive body parts, which, after significant naturpal aging up to e.g. 6 months after pre-ageing treatment, has an excellent hemming performance in T4P temper, in combination with a good paint-bake response.
It is a further object of the invention to provide a method of manufacturing an aluminium alloy sheet product of the AA6000-series, which can be performed in an industrial context, preferably in a continuous annealing line.

DESCRIPTION OF THE INVENTION

These and other objects and further advantages are met or exceeded by the present invention concerning a method of manufacturing an AlMgSi- or AlMgSiCu-alloy sheet product particularly suitable for use in the production of automotive body parts, the method comprising the steps of

(a) providing a rolled aluminium alloy sheet comprising an AA 6000-series alloy;
(b) solution heat treating the rolled aluminium alloy sheet;
(c) quenching said sheet;
(d) pre-ageing said quenched sheet by subjecting it to a heat treatment at a pre-ageing temperature in a range of 65°C to 110°C for a pre-ageing time in a range of 20 min to 70 min.;
(e) cooling, preferably to below 35°C, the pre-aged sheet from the pre-ageing temperature at a cooling rate of more than 3°C/h.

As will be appreciated herein below, unless otherwise indicated, all aluminium alloy designations and temper designations refer to the designations by the Aluminum Association as published e.g. in "International Alloy Designations and Chemical Composition Limits for Rolled Aluminum and Rolled Aluminum Alloys" in January 2018.
For this invention, the term "sheet" or "sheet product" refers to a rolled product having up to 4.0 mm in thickness.
For all descriptions of alloy compositions, all references to percentages are by weight % unless otherwise indicated.
The term "up to" and "up to about", as employed herein, explicitly includes, but is not limited to, the possibility of zero weight-percent of the particular alloying component to which it refers. For example, up to 0.15% Zn may include an alloy having no Zn.
In accordance with the invention, it has been found that the yield strength after a paint-bake simulation consisting of 2% pre-strain and holding at 185°C for 20min, is significantly stabilized by using a higher pre-aging temperature than was normally applied in the industrial scale process, namely by using a pre-aging temperature in the range of 65-110°C, preferably 80-110°C, more preferably 85 to 105°C, wherein the most effective range was found to be between 90-100°C.
The T4P temper here refers to a material that has been solution heat treated and quenched and then subjected to a pre-ageing treatment and then natural aged to provide said T4P temper. The natural ageing occurs as a result of storing at ambient (room) temperature, typically for about 72 hours up to 6 months, until forming or shaping into an automotive body panel, in particular into a three-dimensional automotive panel.
The T64 temper here refers to a T4P temper material that has been deformed in tension by 2% pre-strain followed by a 20 minutes treatment at 185°C to represent the forming plus paint curing treatment typically experienced by automotive panels.
The difference in strength between T64 and T4P is often referred to as the paint-bake response.
Although not intended to be bound to any scientific theory, it is believed that the pre-aging temperature of the invention is particularly advantageous because it leads to the formation of a particular type of nano-clusters, named C2-clusters, which enhance the formation of the β"-phase during the paint-bake treatment, and therefore increase the paint bake response. The precipitation of the β"-phase is believed to be responsible for the strengthening of the subject aluminium alloy sheet during paint-baking. These advantageous C2 clusters form between 60-130°C. Therefore, pre-ageing temperatures in this range are advantageous. Further, it is believed that in the temperature range between about 30°C and about 60°C, to a lesser extent up to 80°C, a Si-rich cluster named C1-cluster is formed, which appears detrimental to the paint-bake response, since it does not dissolve at the paint-bake temperature. 60°C<T<80°C is the overlapping range where both C1 and C2 clusters form (T stands for temperature), and this range needs to be treated with care. Therefore, longer pre-ageing times might be necessary when pre-ageing within this temperature range (i.e. at 65°C to 80°C) to achieve an effective pre-ageing treatment and a good paint bake response. The pre-ageing temperatures in the range 80-110°C appear to require shorter soak times (e.g. less than 60 min. or even less than 45min.), which is advantageous in an industrial context. Owing to the formation of detrimental Si-rich C1 clusters in the range 30°C<T<60°C, it appears advantageous to avoid short stay of the material at such temperatures.
In the prior art, often long pre-aging times of two hours or more have been used, and which will result in a good paint-bake response. However, when also hemming performance and bendability in the T4P condition after natural aging for several weeks is considered, it has been found that a pre-aging time of more than 70 min, or even more than 1h, is too long as it degrades the hemming performance in T4P condition. Therefore, the pre-aging time is limited to 20-70 min according to the invention, preferably 25 to 60min. A preferred lower limit is about 25 min, and more preferably about 30 min, and most preferably about 35 min. A preferred upper-limit is 60 min., more preferably 55 min., and most preferably 45 min.
The higher pre-aging temperature may lead to a deterioration in hemming performance. To compensate for this, it has been found that it is advantageous to cool the sheet product from the pre-aging temperature at a relatively high cooling rate of more than 3°C/h. In the prior art, such as disclosed in patent document WO2016/196166-A1 , the sheet product is directly coiled after the pre-aging step and placed in an ambient environment. If on an industrial-scale a coil of aluminium alloy sheet, which weighs up to about 10t, is left to cool down in natural air cooling, the cooling rate will be very slow, e.g. around or even below 1°C/h, as is disclosed for example in patent document US 2016/0047021 A1 . This is the usual state of the art in industrial scale production of automotive body sheet. It has been found that, surprisingly, the properties of the final sheet product regarding bendability and hemming performance are significantly improved by applying a faster cooling rate, for example; by forced convection air cooling. In this embodiment, the coiled sheet is subjected to a constant airflow generated by a fan. Such forced air cooling will result in cooling rates in a range of 3°C/h to 10°C/h, preferably in a range of about 3.5°C-8°C/h. In other embodiments of the invention, water cooling is used after the pre-aging treatment, which results in cooling rates of >50°C/h, preferably in a range of 60-150°C/h, more preferred in a range of 80-120°C/h, most preferred in a range of 90-110°C/h. A possible reason for the advantage of using a forced cooling after pre-aging rather than natural cooling, is that the detrimental temperature zone between 30-60°C, in which the C1 clusters form, is passed through relatively quickly, avoiding long dwell times in this temperature range.
According to a preferred embodiment, the sheet product is cooled from the pre-aging temperature with the above-described cooling rate down to below 35°C, preferably to below 30°C or to ambient temperature. Thereby, holding of the sheet product in the temperature range where C1 clusters form is avoided.
Preferably, the cooling time from pre-aging temperature should be less than 20h, preferably less than 10h, more preferred less than 5h. Such shorter cooling time will also result in an improvement in the hemming performance in the T4P condition after natural aging for a significant amount of time.
According to an embodiment of the invention, it has been found that in some processes, the hemming performance in T4P condition may be further improved by modifying also the quenching step after solution heat treatment, namely by pursuing the quench/cooling of the sheet to below 45°C, preferably to below 35°C, more preferred to below 30°C and most preferred to ambient temperature or room temperature. In a continuous annealing line (CAL) used in industrial scale production of automotive body sheets so far, the sheet product is often not cooled down to room temperature, but to around 60°C-70°C. This was supported by prior research, which placed the most emphasis on the quenching rate rather than the exit temperature of the quenching operation. It has now been found that the quench rate has no significant influence on mechanical properties as long as the exit temperature of the quenching operation is sufficiently low, namely below 45°C, preferably below 35°C. Also here, this may be related to the formation of adverse C1 clusters in the temperature range of 30-80°C, which is significantly reduced by quenching down to below 45°C. It has been found that using such extended quenching has a positive effect on the hemming behaviour in T4P after natural aging for 6 months.
In a preferred embodiment, quenching from solution heat treatment temperature is done by water-quenching, which allows cooling to the desired low temperatures.
The method steps (b) to (d) are preferably performed on a continuous annealing line ("CAL"), i.e. the rolled aluminium alloy is provided in the coiled state, and is passed through the CAL. In other words, the sequential steps of solution heat treatment, quenching and pre-aging are performed not batch-wise but in-line in a continuous processing line.
For the cooling from the pre-aging temperature, in a preferred embodiment the sheet product is coiled after passing the continuous annealing line, and wherein the cooling step of the aluminium sheet is thus performed in the coiled state.
In regular industrial production lines, often the quenched sheet will be subjected to levelling and/or etching and/or passivation prior to the pre-aging step. Since these process steps take some time, in most embodiments, there will be a time delay between quenching and the pre-aging step. But this delay should be as short as possible, since the natural aging will start directly after quenching. Accordingly, the pre-aging step is preferably started within 30 minutes, more preferably within 20 minutes after quenching, more preferred after 15 minutes and most preferred within 5-12 minutes following the quenching step.
In useful embodiments, the aluminium alloy sheet is passed through the continuous annealing line at a line speed of up to 120 m/min, wherein the higher line speeds are preferred, preferably in a range of 50 to 100 m/min, more preferably 50 to 90 m/min, as experiments have shown that better hemming performance in combination with good bake response can be realized at relative high line speeds. The reason may be the reduced natural aging time between quenching and pre-aging. The method according to the invention guarantees the properties at both extreme speed limits.
In useful embodiments, solution heat treatment is performed at a temperature in a range of 510°C to 580°C, preferably 520-565°C, more preferred about 530°C to 560°C, wherein a solution heat treatment temperature of about 550±5°C is most preferred. Thus, a relatively high solution heat treatment temperature has been found useful, since it leads to a more complete solid dissolution of elemental excess Si as well as Mg₂Si, and this will result in the good hardening response of the aluminium alloy during paint-baking.
The AlSiMg- or AlMgSiCu-alloy can be provided as an ingot or slab for fabrication into rolling feedstock using casting techniques regular in the art for cast products, e.g. DC-casting, EMC-casting, and preferably having an ingot thickness in a range of about 220 mm or more, e.g. 400 mm, 500 mm or 600 mm. In another embodiment thin gauge slabs resulting from continuous casting, e.g. belt casters or roll casters, also may be used, and having a thickness of up to about 40 mm. After casting the rolling feedstock, the thick as-cast ingot is commonly scalped to remove segregation zones near the cast surface of the ingot. The cast ingots are subjected to a homogenization treatment by heating the ingot to a temperature of above 540°C, but at a temperature lower than the solidus temperature of the subject alloy; maintaining the ingot at this temperature for at least about 4 hours, and preferably for at least about 10 hours. After suitable pre-heating, the ingots are hot-rolled and subsequently cold-rolled. Optionally, there may be an intermediate annealing step between two cold-rolling steps. After the last cold-rolling step, the rolled sheet has a final thickness of usually 0.5 to 4.0 mm, and preferably in a range of 0.7 mm to 2.0 mm, for example a thickness of about 1.0 mm or about 1.2 mm. The cold-rolled band or sheet is then further processed according to the method of this invention.
In preferred embodiments, the provided rolled aluminium alloy sheet product, comprising an AA6000-series has a chemical composition closely associated with the known AA6016 or AA6016A series, or modifications thereof.

The rolled aluminium sheet may comprise a single layer of the AA6000-series alloy. In an alternative embodiment, the rolled aluminium sheet may comprise several layers. At least one layer is made of an aluminium alloy of the AA6000-series, and preferably having the following composition, in weight %:

Si	0.75 -1.5, preferably 0.9-1.4, most preferred 1.0-1.30,
Mg	0.2-0.8, preferably 0.25-0.7, most preferred 0.4 -0.60,
Fe	<0.5, preferably <0.35, most preferred 0.10 -0.3,
Mn	<0.25, preferably 0.03-0.20, most preferred 0.03-0.15,
Cu	<0.40, preferably <0.25, more preferably <0.20,
Cr	up to 0.10,
Zn	up to 0.25,
Ti	up to 0.10,

balance aluminium and impurities, each max 0.05%, in total max. 0.15 %.

The purposive addition of Mg and Si strengthens the alloy due to precipitation hardening of elemental Si and Mg₂Si. In order to provide a sufficient strength level, the sheet product according to the invention preferably has a Si content of at least 0.75%, preferably 0.8%, preferably at least 0.9%, and most preferred at least 1.0%. A preferred upper limit for the Si content is 1.4%, more preferred 1.30%.
Substantially for the same reason as for the Si content, the Mg content should be at least 0.2%, preferably at least 0.25% and more preferred at least 0.4%, in order to provide sufficient strength to the sheet product after the paint-bake cycle. A preferred upper-limit for the Mg-content is 0.7%, and more preferably 0.60%.
In some embodiments, the amount of iron (Fe) is carefully controlled in order to provide an improved hemming performance compared to aluminium alloys with higher Fe levels, and having otherwise the same composition. The Fe content in the alloy sheet product is preferably <0.35%, more preferred <0.3% and most preferred 0.10-0.3%.
Cu can be purposively present in the sheet product to a level of <0.40, but preferably should not exceed 0.25 in order to maintain a good corrosion performance. Preferably, Cu is present up to 0.20%, more preferred up to 0.15%. In useful embodiments, Cu is purposefully added to be present in a range of 0.05-0.20%, preferably 0.05-0.17%. It appears that Cu is affecting the precipitation sequence of the hardening Mg₂Si phase, leading to a favourable stabilisation of the pre-aging properties and a higher paint-bake response.
Mn is added to the alloy sheet product for grain size control to improve the formability of the sheet product. In particular the elongation is improved due to the reduced fraction of constituent particles. The Mn level should be present in a range of up to 0.25%, preferably up to 0.20%, and more preferably in a range of 0.01% to 0.15%. A preferred lower-limit for the Mn content is about 0.03%, and more preferably about 0.05%.
Cr can be present up to 0.10%. Cr is preferentially avoided in the sheet product as it may prevent full recrystallization of the sheet product. Preferably it is tolerated up to 0.04%, and is preferably less than 0.03%, and more preferably less than 0.02%.
Also each of vanadium (V) and zirconium (Zr) are preferentially avoided in the sheet product as they may prevent full recrystallization of the sheet product. Such elements are costly and/or form detrimental intermetallic particles in the aluminium alloy. Thus, the sheet product generally includes not greater than 0.03% V and not greater than 0.03% Zr. In a preferred embodiment, the sheet product includes V only up to 0.02%. In a preferred embodiment, the sheet product includes Zr only up to 0.02%.
Zn may optionally be included in the alloy, and in an amount up to about 0.25%. Zinc may be present in scrap, and its removal may be costly. In one embodiment, the alloy includes not greater than 0.10% Zn, and in a preferred embodiment the alloy includes not greater than 0.05% Zn.
Ti can be added to the sheet product amongst others for grain refiner purposes during casting of the alloy ingots. The addition of Ti should not exceed 0.10%, and preferably it should not exceed about 0.05%. A preferred lower limit for the Ti addition is about 0.008%, and can be added as a sole element or with either boron or carbon as known in the art serving as a casting aid, for grain size control.
Unavoidable impurities may be present up to 0.05% each, and in total up to 0.25%, preferably up to 0.15%, and the balance being aluminium.

According to an embodiment of the invention, the provided rolled aluminium alloy sheet comprising an AA6000-series alloy has a composition consisting of:

balance aluminium and impurities, each max 0.05%, in total max. 0.15 %. The alloy may have preferred narrower compositional ranges as herein described and claimed.

The invention is also directed to an aluminium alloy sheet product, particularly suitable for use in the production of automotive body parts, and which has been manufactured by the herein described and claimed method according to the invention or one of its embodiment. Such sheet product has been found to have a hemming performance of 3 or more, preferably 3 ½ or more after six months of natural aging, which corresponds to the T4P temper. Once this 6 months naturally aged sheet has been subjected to a simulated paint-baking cycle of 2% pre-strain and heating at 185°C for 20 minutes, it exhibits a paint bake response of 80 MPa or more, preferably of 85 MPa or more.
Accordingly, the aluminium alloy sheet product produced according to the invention has excellent forming, and in particular hemming properties after extended natural aging for more than two months and therefore can easily be processed by or for an automobile manufacturer into an automotive body part by means of forming (e.g. by deep-drawing, pressing, or stamping). Once such formed automotive body part is subjected to the paint-bake cycle, the yield strength is advantageously increased by at least 80MPa, which may result in a final yield strength of >210MPa, in some embodiments even >215 M;Pa, and in the better examples >230 MPa. This significantly exceeds the requirement of >=200MPa.
The product of the invention can advantageously be used as an automotive panel, ideally an outer panel, with guaranteed paint-bake response, hemming and bending performance for 6 months after CAL-treatment, pre-ageing and natural ageing.

BRIEF DESCRIPTION OF THE FIGURES

Fig. 1: is a flow-diagram of the usual process steps used in manufacturing a Al-SiMg alloy rolled sheet product for use in the production of automotive body parts;
Fig 2: is a schematic temperature vs. time diagram, illustrating the heat treatments of the CAL according to the invention.

DESCRIPTION OF EMBODIMENTS

The invention will now be described with reference to an advantageous non-limiting embodiment with reference to the attached figures.
Figure 1 illustrates an embodiment of the processing steps used in the manufacturing of AA6000-series aluminium alloys to produce automotive body sheets according to the invention.
In the first step 2, ingots are cast using DC or EMC casting. The alloy is preferably of the AA6016-series, although the invention may also find use for e.g. AA6014, AA6081, AA6451, or AA6005A-type of alloys.
The cast ingots are subject to scalping and a usual homogenization treatment in step 4. The homogenized ingot is hot-rolled in step 6, and the hot-rolled plate is passed to the cold-rolling mill. A first cold-rolling step 8 is followed by an intermediate annealing step 10 and afterwards, a second and final cold rolling step 12 is performed. There may be variations in the number of cold-rolling and intermediate annealing steps, in particular, there may be further intermediate annealing and cold-rolling steps. After the last cold-rolling step, the sheet is in the form of a band having a thickness of e.g. 0.5-4.0 mm, typically 0.8-1.5mm. The rolled aluminium alloy sheet provided to the solution heat-treatment and pre-aging treatment according to this invention will usually be made of one monolithic layer, but the invention is equally applicable to multi-layered sheet products, wherein one or all of the layers are of the AA6000-series.
After step 12, the rolled aluminium sheet is typically supplied to a continuous annealing line ("CAL"). In the CAL, the steps of solution heat treatment and quenching to below 35°C are performed in step 14. Afterwards, but preferably in the same continuous annealing line, the quenched sheet product is subjected to levelling and in useful embodiments also to etching and passivation in step 16. These process steps are carried out as known in the art, as they are useful in improving the surface properties of the automotive body sheet.
Following this step 16, the sheet product is heated to the pre-aged temperature in step 18, preferably coiled at pre-aging temperature, soaked at the pre-aging temperature for the prescribed pre-aging time, and subjected to a controlled cooling step resulting in a cooling rate of >3°C/h in step 18.
The temperature profile of the inventive manufacturing method is illustrated in Fig. 2, which shows a temperature profile of a continuous annealing line according to an embodiment of the invention. At 20, the rolled sheet product is heated up to preferably above 540°C in the solution heat treatment step. Such high temperature is advantageous as it results in the highest possible rate of dissolution of Mg₂Si and Si. The shaded area 24 between 400-540°C shows the temperature range in which Si is dissolved in solid solution. In the range 22 between 400-480°C Mg₂Si is dissolved. In the temperature range between 300-400°C, which is dotted and marked with 26, Mg₂Si and Si are dissolved in the cold-rolled material. It follows that above 540°C, the dissolution of Mg₂Si and Si is complete.
The material is held at the SHT-temperature for a short time, e.g. for less than 1 minutes, preferably less than 30 seconds, and quenched 28 preferably by water quenching, in this illustrative example to below 30°C. The quenching rate is as usual in the art, namely about 200-400°C/s.
The advantage of quenching to below 45°C and more preferably to below 35°C or even below 30°C is illustrated in Fig.2 by the various zones 38-46, in which the C1 and C2 clusters form: in the temperature range 46 between 30-80°C, the C1 clusters tend to form, which are detrimental to the paint bake response, since they delay precipitation β"-phase. This effect is believed to be strongest in the middle of this range, between about 45°C and 60°C. Hence, this temperature range may be called the "unsafe zone". Accordingly, it is advantageous to quench to below 45°C in step 28.
In the temperature range 44 above 60°C up to 130°C, the C2 clusters form, which have been shown to have a positive effect on the properties of the sheet after natural aging, including paint-bake response (PBR). Accordingly, the zone 44 between 80-130°C, in which only C2 clusters form, is designated as "safe zone", and is a useful temperature range for the pre-ageing treatment. The zone 40 between 60-80°C is characterised by the formation of both C1 and C2 clusters, and therefore may still be used with advantageous effects for pre-ageing. At these comparatively low temperatures, longer soak times are preferred according to the invention, e.g. around 40 min or 50 min to 70 min.
After quenching, the sheet product is held at ambient temperature 30 for less than 30 minutes, while additional process steps such as levelling, etching and/or passivation are performed. These process steps typically take place at a temperature of e.g. between 20-30°C. After completion of these steps, and as soon as feasible after quenching, the sheet product is heated to the pre-aging temperature, in this case between 80-110°C in step 32. The sheet product is held there in step 34 for a pre-aging time of 20 min or more, however, preferably less than 70 min, wherein the holding at pre-aging temperature may take place in the uncoiled or already coiled state. After the soaking time at pre-aging temperature, the sheet, usually the coiled sheet, is subjected to forced convection cooling 36 to ambient temperature, resulting in a cooling rate of 3-150°C/h, and a cooling time of 20 hours or less.

EXAMPLES

The invention will now be illustrated with reference to non-limiting experimental examples.
Several coils of sheet products of 1.2 mm final gauge were produced in industrial scale under various processing conditions, some according to the prior art and some according to different embodiments of the invention. For each case, the provided sheet product consisted of an aluminium alloy having the following composition:

Si 1.1%

Mg 0.4%

Fe 0.25%

Mn 0.11%

Cu 0.08%

Ti 0.02%

Cr 0.02%,

balance impurities and aluminium.

The process parameters of the various coils and samples, which are named C1 and C2 for the comparison samples, and S1-S4 for the inventive samples are shown in Table 1. The resulting mechanical properties are shown in Table 2 and have been measured according to international standard ISO 6892-1 (second edition, July 2016).

Table 1: Processing parameters of sample coils

Coil	Sample	Process parameters
		SHT	Quench	PAT	PA-time	Cooling rate
		(°C)	type	(°C)	(min)	(°C/h)
Coil #1	C1 (comp.)	550	Standard	60	60	1-1.5
Coil #2	C2 (comp.)	540	Standard	95	60	1-1.5
Coil #3	S1 (inv.)	550	Standard	96	30	100
Coil #3	S2 (inv.)	550	Extended	96	30	100
Coil #4	S3 (inv.)	550	Extended	93	45	3,2
Coil #5	S4 (inv.)	550	Extended	100	30	3,8

Table 2: Mechanical properties of samples

Sample	Mechanical properties
	Re-T4P (25 d)	Re-T4P (6 m)	Re-T64 (25 d)	Re-T64 (6 m)	PBR	drop of Re T64 in 5 m	Hem.-L T4P (6 m)
	(MPa)	(MPa)	(MPa)	(MPa)	(MPa)	(MPa)
C1 (comp.)	115	125	226	202	77	24	2
C2 (comp.)	120	130	221	214	84	7	1¾
S1 (inv.)	114	127	215	214	87	1	3¼
S2 (inv.)	117	131	218	217	86	1	3½
S3 (inv.)	126	136	229	224	88	5	3¼
S4 (inv.)	123	135	231	226	91	5	3¼

The solution heat treatment temperature was 540°C for comparison sample C2 and otherwise 550°C. The quench type is either standard or extended, wherein standard relates to the standard quenching procedure wherein the quench exit temperature is above 60°C. The extended quench type relates to an embodiment of the inventive quenching step with an exit temperature after quenching of below 30°C. After quenching, the usual passivation step was performed. "PAT" stands for pre-aging temperature, and "PA-time" for the holding time at PAT. The cooling rate stands for the cooling rate from the pre-ageing temperature after the PA-time. The coils #4 and #5 and coiled sheet product samples S3 and S4, which were cooled at a cooling rate of 3.2°C/h and 3.8°C/h, respectively, were cooled in a forced convection cooling chamber, whereas coil #3 and samples S1 and S2 having a cooling rate of about 100°C/h were cooled by water spraying.
The mechanical properties of the materials are listed in Table 2. The yield strength Re was always measured in the transverse direction and is given in T4 temper after the pre-aging treatment followed by 25 days of natural aging (25d), and 6 months of natural aging (6m). The hemming performance in L-direction (rolling direction) was measured also in T4P condition after 6 months of natural aging. The hemming performance was tested via a flat hemming test, by bending the samples 180° with a bending radius of 0.0mm as described in ASTM Norm E290-97-A and followed by a visual assessment. A score was given according to the following rating:
Rating "5" represents no visual defects, "4" mild surface roughening, "3" severe surface roughening, "2" small surface cracks, and "1" represents continuous surface cracks, and whereby a further sub-rating of for example 3¼, 3½ and 3¾ is used. A rating of 2½ or below is unacceptable for the preferred intended application as an automotive outer panel.
The samples were subjected to a simulated paint-bake cycle, which consisted of 2% stretch and soaking at 185°C for 20min, resulting in a T64 temper. The tensile tests in T64 temper were done in transverse direction and the increase in Re during the paint-bake cycle is given as the paint-bake response (PBR).
Coil #1 with sample C1 represents a sheet product produced with the normal state-of-the-art processing conditions, including a standard quench and a pre-aging treatment at 60°C for 60 minutes, followed by the usual natural convection cooling in the coiled state, and which results in a slow cooling rate of 1-1.5°C/h. As one can see, the C1 sample has a low Re-T6 and low paint-bake response. Also, the Re-T6 drops significantly by 24MPa during the 6 months aging. The hemming performance at 2 is not acceptable. By raising the pre-aging temperature to 95°C, as was done for coil #2 and sample C2, the paint-bake response is increased significantly to 84 MPa, and the final yield strength Re-T64 after 6 months ageing is 214 MPa. However, the hemming performance is degraded at the same time to 1¾, which is below acceptance level and even lower than for sample C1.
Coil #3 and sample S1 were processed according to the invention, with a standard quench and a cooling rate of 100°C after pre-ageing. The pre-ageing treatment was done at 96°C for 30 minutes. As can be seen from table 2, the hemming performance is significantly improved to 3¼ in comparison with sample C2, while the PBR is even higher at 87 MPa. It is also noticeable that the drop of Re-T64 in the 5 months between the two measurements (Re-T64 (25d) and Re-T64 (6 m)) is negligible at 1 MPa. Hence, this process results in an excellent stability in mechanical properties, in particular of the PBR, in combination with a very good hemming performance in T4P condition after significant natural ageing.
Sample S2 of the same coil was processed in the same way as sample S1, with the exception that an extended quench was used. As can be seen in Table 2, this leads to a further improvement of the hemming performance in T4P condition from 3¼ to 3½.
For Sample S3, a cooling rate of about 3.2°C/h was realized by forced convection cooling, and it can be observed that the paint bake response is even further increased to 88 MPa, with a very high Re-T64 of 224 MPa. Also the hemming performance is very good at 3¼. Similar values are observed for Sample S4, where a cooling rate of 3.8°C/h after the pre-aging treatment was realized. The pre-aging was done at 100°C for 30 min, and the PBR is even higher at 91 MPa.
The comparison of sample S3 and S4 with S1 and S2 demonstrate that forced convection cooling is adequate to achieve a cooling rate after pre-ageing which is sufficient to realize the advantages of the invention. The comparison with sample C2 shows that natural cooling, however, will not produce satisfactory results, as the cooling rate after pre-ageing is too low. Further, raising the pre-ageing temperature from 60°C to above 65°C to 110°C, preferably to 80-110°C, while adjusting the PA-time, is shown to improve the stability of the mechanical properties during natural ageing, in a particular Re-T64 and PBR.
The invention is not limited to the embodiment described above, which may be varied widely within the scope of the invention as defined by the appended claims.

Claims

A method of manufacturing an AlMgSi alloy sheet product particularly suitable for use in the production of automotive body parts, the method comprising the steps of
(a) providing a rolled aluminium alloy sheet comprising an AA 6000-series alloy;

(b) solution heat treating the rolled aluminium alloy sheet;

(c) quenching said sheet;

(d) pre-ageing said quenched sheet by subjecting it to a heat treatment at a pre-ageing temperature in a range of 65°C to 110°C for a pre-ageing time in a range of 20 min. to 70 min.;

(e) cooling the pre-aged sheet from the pre-ageing temperature at a cooling rate of more than 3 °C/h.
The method of claim 1, wherein cooling of the pre-aged sheet is done by forced convection cooling.
The method of one of the preceding claims, wherein at least the method steps of solution heat treatment, quenching and heating to the pre-ageing temperature are performed while passing said rolled aluminium alloy sheet through a Continuous Annealing Line.
The method of one of the preceding claims, wherein the aluminium alloy sheet is coiled after heating to the pre-ageing temperature, and the heat treatment step at pre-ageing temperature for the pre-ageing time and the cooling step (e) is performed on the coiled sheet product.
The method of one of the preceding claims, wherein the pre-ageing time is 25 to 60 min., preferably 30 to 55 min., most preferred 35 to 45 min.
The method of one of the preceding claims, wherein the pre-ageing temperature is 80°C to 110°C, more preferably 85°C to 105°C, and most preferred 90°C to 100°C.
The method of one of the preceding claims, wherein the pre-aged sheet is cooled from the pre-ageing temperature to below 35°C at a cooling rate of more than 3 °C/h.
The method of one of the preceding claims, wherein the solution heat treatment is carried out at a temperature of 510°C to 580°C, preferably 520°C to 565°C, most preferred 540 to 560°C.
The method of one of the preceding claims, wherein the quenching after solution heat treating is pursued to below 45°C, preferably to below 35°, more preferred to below 30°C, most preferred to ambient temperature.
The method of one of the preceding claims, wherein the pre-ageing treatment is started within 20 minutes, preferably within 15 minutes, after quenching.
The method of one of the preceding claims, wherein the rolled aluminium alloy sheet comprises at least one layer made from an aluminium alloy having the following composition, in wt.%: Si 0.75-1.5 Mg 0.2-0.8 Fe < 0.5 Mn < 0.25 Cu < 0.40 Cr up to 0.10 Zn up to 0.25 Ti up to 0.10

balance aluminium and impurities, each max 0.05 %, in total less than 0.15 wt.%.