EP3662091A1 - 6xxxx-series rolled sheet product with improved formability - Google Patents

Abstract

The invention relates to a rolled 6xxx-series aluminium alloy sheet product comprising of, in wt.%: Mg 0.10-1.3%, Si 0.25-1.4%, Cu up to 0.45%, Mn up to 0.50%, Fe 0.10-0.45%, Sr 0.01-0.05%, and wherein the ratio Fe/Sr is in a range of 4 to 40, Ti up to 0.20%, Cr up to 0.30%, Zr up to 0.20%, V up to 0.20%, Zn up to 0.35%, Sn up to 0.075%, impurities and balance aluminium. The invention further relates to a method of manufacturing the 6xxx-series aluminium alloy sheet product.

Description

6xxxx -series rolled sheet product with improved formability

FIELD OF THE INVENTION

The invention relates to a rolled 6xxx-series aluminium alloy sheet product.

The sheet product is ideally suitable for automotive applications. The invention further relates to a method of manufacturing the 6xxx-series aluminium alloy sheet product. BACKGROUND TO THE INVENTION

Generally, body panels of a vehicle require excellent properties in formability, dent-resistance, paint-bake response, corrosion resistance and surface quality properties. However, the conventional AA5xxx-series alloy sheets have not been favoured because they have low mechanical strength even after press forming and may also exhibit poor surface quality. Therefore, 6xxx-series sheet alloys have been increasingly used. In general, the 6xxx-series alloys provide excellent bake harden- ability after painting and high mechanical strength as a result, thus making it possible to manufacture more thin-gauged and more light-weight sheets in combination with a good corrosion resistance and class-A surface finish. There is a need for aluminium alloy rolled sheet products suitable for use in automotive panels and exhibiting improved formability. DESCRIPTION OF THE INVENTION

As will be appreciated herein below, except as otherwise indicated, aluminium alloy designations and temper designations refer to the Aluminium Association designations in Aluminium Standards and Data and the Registration Records, as pub- lished by the Aluminium Association in 2016 and are well known to the person skilled in the art.

For any description of alloy compositions or preferred alloy compositions, all references to percentages are by weight percent 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 0.35% Zn may include an alloy having no Zn, and thus there may be an absence of such element.

It is an object of the invention to provide a 6xxx-series aluminium alloy sheet product with improved formability.

This and other objects and further advantages are met or exceeded by the present invention and providing a rolled 6xxx-series aluminium alloy sheet product consisting of, in wt.%:

Mg 0.10% to 1 .3%,

Si 0.25% to 1 .4%,

Cu up to 0.45%

Mn up to 0.50%

Fe 0.10% to 0.45%

Sr 0.01 % to 0.05%

and wherein the ratio Fe/Sr is in a range of 4 to 40

Ti up to 0.20%,

Cr up to 0.30%,

Zr up to 0.20%,

V up to 0.20%,

Zn up to 0.35%,

Sn up to 0.075%, other elements and impurities each <0.05%, total <0.15%, balance aluminium.

In accordance with the invention, it has been found that the purposive addition of strontium (Sr) in the defined range in combination with the Fe/Sr ratio offers an improved formability, in particular an increased stretch formability assessed by an Erichsen Dome Height test. In addition, the rolled sheet product in accordance with the invention allows for the presence of higher amounts of Fe while offering a form- ability commonly found in 6xxx-series alloy sheet products having a lower Fe-con- tent. This enables to maintain balance of strength and a good formability while using a higher content of recycled material and thereby increasing the environmental sus- tainability.

Some prior art documents disclosing the addition of strontium (Sr) to wrought aluminium alloys are:

Patent document WO 2005/108633 A2 (Erbsloh) discloses 6xxx-series aluminium alloy having 0.3-0.9% Si, 0.1 -0.5% Mg, up to 0.2% Fe, 0.1 -0.4% Cu, 0.03- 0.2% Mn, 0.01 % Ti, 0.08-0.22% Zr and/or Cr and/or V, up to 0.005% Ag, up to 0.04% Zn, and wherein the ratio (in wt.%) of Si to Mg is 1 .8:1 to 3.3:1 , and the ratio (in wt.%) of Fe to Sr is 3:1 to 5:1 . The strontium addition is to ensure that the alloy can be decoratively anodized and shows no yellowish or cloudy eloxal coating. The strontium is said to alter the Fe-, Zr-, Cr, and/or Fe-containing phases to the extent that they do not cause visible clouding when they are incorporated in the eloxal coating.

Patent document US 3,926,690 (Alcan) discloses that the addition of 0.02-

0.05% of Sr and/or Ca to the AA6063 extrusion alloy in order to promote the formation of the less detrimental alpha-AIFeSi form, having the effect of improving the surface quality of the extrusion at increased extrusion speeds. No effects on mechanical properties are reported.

The paper "Effect of strontium on microstructure and properties of aluminium based extrusion alloy 6061", by F. Paray et al., in Materials Science and Technology, April 1996, Vol. 12, pp.315-322 shown that in this extrusion alloy, the strontium altered the platelike β-AIFeSi phase (AlsFeSi) to the Chinese script alpha-AIFeSi compound (AlsFe2Si). While strontium may shorten the homogenisation process, it has no adverse effects on the mechanical properties of the extruded end products; a slight decrease in tensile strength of the strontium containing alloy was observed.

Effects and reasons for limitations of the alloying elements in the Al-Mg-Si alloy sheet manufactured in accordance with the method of the present invention are described below.

The purposive addition of Mg and Si strengthens the alloy due to precipitation hardening of elemental Si and Mg2Si formed under the co-presence of Mg. In order to provide a sufficient strength level in the sheet product according to the invention, the Si content should be at least 0.25%, and preferably at least 0.50%, and more preferably at least 0.65%. In an embodiment, the Si content is at least 0.75%. A preferred upper-limit for the Si content is 1 .4%, and more preferably 1 .3%. The pres- ence of Si enhances also the formability, and excess Si with respect to Mg promotes a fast paint-bake response.

Substantially for the same reason as for the Si content, the Mg content should be at least 0.10%, and preferably at least 0.20%, and more preferably at least 0.25% to provide sufficient strength to the sheet product. The upper-limit for the Mg content is 1 .3%, and preferably 1 .0%. A too high Mg excess may increase the fraction of undesired second phase particles by the formation of Al-Fe-Mg phases.

In one embodiment, the rolled 6xxx-series aluminium alloy sheet product has a ratio (in wt.%) of Si/Mg of at least 0.90. Preferably, the Si/Mg ratio does not exceed 1 .40, and more preferably it does not exceed 1 .30. This embodiment has also a very good corrosion resistance and high bendability and hemmability. In combination with the addition of Sr, a significant improved Erichsen Dome Height is achieved.

In one embodiment, the rolled 6xxx-series aluminium alloy sheet product has a ratio (in wt.%) of Si/Mg in a range of 2.0 to 7.0. A preferred lower-limit for the Si/Mg ratio is 2.5, and more preferably 3.0, and more preferably 4.0. A preferred upper- limit for the Si/Mg ratio is 6.5. This embodiment has in particular a very good form- ability, more in particular a good Erichsen Dome Height is achieved. In the embodiment wherein the rolled 6xxx-series aluminium alloy sheet product has a ratio (in wt.%) of Si/Mg in a range of 2.0 to 7.0, and with preferred narrow ranges, to provide improved formability, it is preferred that the area fraction of Si- particles having a size of more than 0.35 microns (when observed by Light Optical Microscopy at a magnification of 500x) is less than 0.15%, and preferably less than 0.1 1 %. In a further embodiment, the equivalent average radius of Si-particles having a size of more than 0.35 microns (when observed by Light Optical Microscopy at a magnification of 500x) is less than 1 .4 microns, and preferably less than 1 .3 microns.

Copper (Cu) can be present in the rolled 6xxx-series sheet product to enhance in particular the work hardening behaviour and the paint-bake response, but it should not exceed 0.45%. A preferred upper-limit for the Cu-content is 0.40%, and more preferably 0.30%. In a preferred embodiment, Cu is purposively added in a range of at least 0.02%, and preferably of at least 0.04%.

Iron (Fe) should remain within a range of 0.10% to 0.45%. A too low Fe-content may lead to undesired grain growth in the final sheet product adversely affecting several formability characteristics. In addition, the effectivity of the Sr addition is less at a low Fe content. A too high Fe-content has an adverse effect on the formability and mechanical properties and is difficult to compensate by the purposive addition of Sr. A preferred lower-limit for the Fe-content is 0.18%. In an embodiment, the Fe- content is at least 0.20%. In an embodiment, the Fe-content is at least 0.22%. A preferred upper-limit for the Fe-content is 0.40%, and more preferably 0.35%.

The strontium (Sr) content must be within a range of 0.01 % and 0.05%, and furthermore, the ratio (in wt.%) of Fe/Sr is in a range of 4 to 40. A preferred upper- limit for the Sr content is 0.045%, and more preferably 0.04%. A preferred lower- limit for the Sr-content is 0.02%, and more preferably 0.025%.

The purposive addition of Sr decreases the area fraction, the number density, and the size (circular equivalent average radius) of coarse Si particles (>0.35 micron), and also the area fraction, the number density, and the size (circular equivalent average radius) of coarse Mg2Si particles (>0.35 micron). The addition of Sr decreases the grain size without increasing the size of the fine second phase particles (these are particles <1 micron diameter that give contrast in Light Optical Microscopy at a magnification of 1000x) and of Fe-bearing particles. This contributes to an increased formability of the 6xxx-series aluminium sheet material, in particular the stretch formability as indicated, for example, by the Erichsen Dome Height and also increases the elongation.

A preferred lower-limit for the Fe/Sr ratio is 5, and a more preferred lower limit is 6. A preferred upper-limit for the Fe/Sr ratio is 20, and more preferably 15. Any Sr is preferable added to the aluminium alloy in the form of a master-alloy, e.g. AISr3.5 or AISr5 or AlSrl 0, prior to casting of the 6xxx-series alloy into rolling feedstock. Strontium is not a common alloying element in rolled 6xxx-series aluminium alloy products, and consequently, the Sr level in any scrap thereof is very low. Often, the Sr level is not standard measured for scrap material, and if present, the Sr level is commonly well below 0.005%, and more typically below 0.001 %.

Each of Mn, Cr, V, and Zr could be present to control the grain size in the rolled aluminium alloy sheet product.

In a preferred embodiment, at least Mn is present in a range of 0.02% to 0.50%. A preferred lower-limit for the Mn content is about 0.05%. A preferred upper-limit for the Mn content is about 0.20%, and more preferably 0.15%, and more preferably 0.10%. Mn is added for grain size control. A too high addition of Mn may interfere with the positive action of the Sr addition.

In a preferred embodiment, there is a purposive addition of Cr in a range of 0.01 % to 0.30%. A preferred upper-limit for the Cr addition is about 0.25%, and more preferably about 0.20%.

In a preferred embodiment, there is a purposive addition of at least Mn in combination with Cr.

Also, each of vanadium (V) and zirconium (Zr), each up to 0.20%, can be added to control the grain size in the final sheet product. In a preferred embodiment, these are preferentially avoided in the rolled aluminium alloy 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 rolled aluminium alloy 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%, and more preferably up to 0.005%. In a preferred embodiment, the sheet product includes Zr only up to 0.02%, and more preferably only up to 0.01 %. Zn is an impurity element that can be tolerated up to about 0.35% and is preferably as low as possible, for example 0.20% or less, and more preferably 0.10% or less.

The addition of tin (Sn) may assist in stabilising the mechanical properties in T4 temper. When added, a preferred addition of Sn is in a range of 0.005% to 0.075%, and more preferably in a range of 0.01 % to 0.06%.

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 about 0.20%, and preferably, it should not exceed about 0.10%. A preferred lower limit for the Ti addition is about 0.01 %, and typically, a preferred upper-limit for Ti is about 0.05%. The Ti can be added as a sole element or with either boron or carbon serving as a casting aid for grain size control.

Unavoidable impurities can be present up to 0.05% each, and a total of 0.15%, the balance is made with aluminium.

In the rolled 6xxx-series aluminium alloy product, there is no purposive addition of elements like In, Th, Er, Sb, Hf, La, Ce, Sm. These are unusual impurities in 6xxx- series alloys, and preferably for each of these elements their presence, if any, is up to 0.005% maximum. In an embodiment, the rolled 6xxx-series aluminium alloy product in a T4 temper has a tensile strength (R_m) of 200 MPa or more and a yield point (Rpo.2) of 90 MPa or more when measured within 30 days after solution heat treatment and quench. In an embodiment, the yield point (Rpo.2) is less than 130 MPa after 6 months storage at ambient (room) temperature. In a further embodiment, it has an elongation at break (Asomm) of at least 24%, and a uniform elongation (A_g) of at least 20%.

In an embodiment, the rolled 6xxx-series aluminium alloy product in a T4 condition and having a sheet material thickness of 1 mm has an Erichsen Dome Height of at least 9.0 mm, and preferably of at least 9.2 mm, when tested in accordance with EN ISO 20482 (July 2003). The 6xxx-series aluminium alloy according to this invention can be provided as an ingot or slab for fabrication into rolling feedstock using semi-continuous casting techniques regular in the art for cast products, e.g. direct chill DC-casting and electro-magnetic 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 semi-continuous as-cast ingot is commonly scalped to remove segregation zones near the cast surface of the ingot.

Next homogenisation should be performed at a temperature of 450°C or more.

If the homogenisation temperature is less than 450°C, reduction of ingot segregation and heterogeneity may be insufficient. This results in insufficient dissolution of Mg2Si components which contribute to the strength, and whereby formability may be decreased. Homogenisation is preferably performed at a temperature of 480°C or more, more preferably at least one homogenisation step is performed at a temperature range of 540°C to 580°C. The heat-up rates that can be applied are those which are regular in the art.

The soaking times for homogenisation should be at least about 2 hours, and more preferably at least about 5 hours. A preferred upper-limit for the homogenisa- tion soaking time is about 48 hours and more preferably about 24 hours.

The hot rolling practice comprises a first hot rolling operation wherein the heated feedstock is subjected to breakdown hot rolling in one or more passes using reversing or non-reversing mill stands that serve to reduce the thickness of the rolling feedstock or ingot to an intermediate gauge range of 15 mm to 40 mm, and preferably of 15 to 35 mm. The breakdown rolling starts preferably at a temperature in the range of about 460°C to 510°C, and more preferably of 470°C to 500°C. The hot-mill process temperature should be controlled such that after the last rolling pass the hot-mill exit temperature of the feedstock is in a range of about 370°C to 480°C. A more preferred lower-limit is about 380°C. A more preferred upper-limit is about 450°C, and more preferably 430°C.

Next after the breakdown hot rolling, the feedstock is supplied to a mill for hot finish rolling in one or more passes to a final gauge in the range of 3 mm to 15 mm, for example, 7 mm or 10 mm. The hot finishing rolling operation can be done, for example, using a reverse mill or a tandem mill. Overall, the thickness of the rolling feedstock or ingot is typically reduced by at least about 65%, and more typically in the range of 80% to 97%. The average temperature of the hot rolled feedstock when the feedstock is inputted into the hot finish rolling process is maintained preferably at a temperature of 370°C to 480°C. A more preferred lower-limit is about 400°C. A more preferred upper-limit is about 450°C.

Control of the finish hot-mill exit temperature of the rolling feedstock is important to arrive at the desired balance of metallurgical properties. The hot-mill tem- perature should be controlled such that after the last rolling pass the hot-mill exit temperature of the feedstock is in a range of about 300°C to 400°C. A preferred lower-limit is about 310°C. A preferred upper-limit is about 380°C, and more preferably about 360°C. A too low or too high exit-temperature of the hot rolled feedstock adversely affects the formability properties of the final product.

Following the last hot-rolling step, the hot-rolled feedstock at final gauge is cooled to below 200°C, more typically to below 100°C, and preferably to ambient temperature. In a preferred embodiment, the cooling of the hot-rolled feedstock at final gauge from hot-mill exit temperature is by the immediately coiling of the hot- rolled feedstock and allowing it to cool in an ambient environment to ambient tem- perature and stored.

In a next step, the hot rolled material is further down gauged by cold rolling to final gauge by applying in one or more rolling steps a total cold rolling degree of at least 40%, preferably of at least 60%.

Optionally during the cold rolling operation, a recrystallization annealing (con- tinuous or batch) can be applied to the cold-rolled product at an intermediate gauge. The annealing temperature is in the range of 360°C to 580°C to achieve recrystali- sation in the cold-rolled product which influences the crystallographic texture development. A preferred lower-limit for the annealing temperature is 380°C, and more preferably 400°C. A preferred upper-limit for the annealing temperature is 500°C, and more preferably 460°C.

Following the optional intermediate annealing heat treatment, the feedstock is cold rolled in one or more cold rolling steps to a final gauge in a range of 0.7 mm to 4.0 mm. A preferred upper-limit for the sheet thickness is 3.0 mm and more preferably 2.5 mm.

In an embodiment of the method, the cold rolled aluminium sheet product at final gauge is solution heat treated at a temperature and for a time such that sub- stantial amounts of Mg2Si and Si, if any, are dissolved into solid solution. The solution heat-treatment temperature is at least 500°C, and is preferably in a range of 520°C to 570°C, and more preferably in the range of 530°C to 565°C, and is more preferably just above the solvus temperature of the Mg2Si and Si phases to further improve formability characteristics of the aluminium alloy sheet product. After the solution heat treating, the sheet is quenched, e.g., by means of water such as cold water quenching or cold water spray quenching. By these processing steps, the main alloying elements Mg, Si and Cu are mostly dissolved during SHT and retained in solid solution by the quenching operation leading to a good formability and control of the yield strength and the bake hardening behaviour. The evolution of the micro- structure at ambient (room) temperature brings the sheet material from a W (as quenched) to a T4 condition.

In an embodiment, following the solution heat treatment and quenching of the sheet product, the sheet product is subjected to artificial ageing or pre-ageing and then natural ageing for 72 hours or longer prior to forming into, e.g., a three-dimen- sional shaped or formed automotive body member. The pre-ageing is preferably performed in a continuous annealing line immediately following the solution heat treatment and quenching by heating up to a temperature in a range of 50°C to 130°C. The pre-ageing treatment provides in time more stable mechanical properties of the sheet product before and after being subjected to a paint bake cycle as well an increased paint-bake response.

In an embodiment, following the solution heat treatment and quenching of the sheet product, the sheet product is subjected to natural ageing for 72 hours to 6 months, optionally even longer, prior to shaping or forming into, e.g., a three-dimensional shaped or formed automotive body member.

Forming operations into three-dimensional shapes includes deep-drawing, pressing, stamping, and stretch forming. Following the forming operation, the formed part may be made part of an assembly of other metal components as regular in the art for manufacturing vehicle components, and subjected to a paint bake operation to cure any paint or lacquer layer applied. The paint bake operation or cycle comprises one or more sequential short heat treatment in the range of 140°C to 210°C for a period of 10 to less than 40 minutes, and typically of less than 30 minutes. A typical paint bake cycle would comprise a first heat treatment of 180°C@20 minutes, cooling to ambient temperature, then 160°C@20 minutes and cooling to ambient temperature. In dependence of the OEM, such a paint bake cycle may comprise of 2 to 5 sequential steps and includes drying steps.

In an embodiment, the rolled 6xxx-series aluminium sheet products according to this invention are cast via continuous casting, e.g., belt casters or roll casters, and have a feedstock thickness of up to about 40 mm. Downstream of the continuous casting operation, the product can be rolled (hot and/or cold), optionally annealed (e.g., between hot rolling and any cold rolling steps), solution heat treated and quenched, optionally cold worked (post-solution heat treatment) or natural aged and optionally also artificially aged, and all these steps may occur in-line or off-line relative to the continuous casting step. The artificially aged product can be painted (e.g., for an automobile part), and may thus be subjected to a paint-bake cycle.

The rolled 6xxx-series aluminium alloy sheet product according to this invention is ideally suitable for manufacturing formed automotive body members. A formed automotive body member includes bumpers, doors, hoods, roofs, trunk lids, fenders, floors, wheels and other portions of an automotive or vehicle body such as body-in-white (e.g., pillars, reinforcements) applications. Due to its combination of excellent deep drawing and stretch forming properties, the rolled 6xxx-series aluminium alloy sheet product is also perfectly suited to produce also inner door panels, wheel arch inner panels, and side panels, spare wheel carrier panels, and similar panels with a high deep drawing height.

The invention will now be illustrated with reference to non-limiting embodiments according to the invention. EXAMPLE 1 .

Six ingots having the dimensions of 430mmx140mmx2000mm have been DC- cast and the six alloy compositions are listed in Table 1 , and whereby alloys 3 and 6 are according to the present invention and having an Fe/Sr-ratio of respectively 6.8 and 7.5.

Each ingot has been homogenised for 10 hours at about 560°C and hot rolled from 80 mm to 10 mm. The hot-mill entry temperature was about 550°C. Following hot rolling warm coiling and self-annealing was simulated in a furnace. Next, the sheet products were cold rolled from 10 mm to 3.0 mm, followed by a batch interan- nealing at 380°C@2 hrs, and then cold rolled to a final gauge of 1 .0 mm. At final gauge, the sheet products have been solution heat-treated at 560°C for about 1 minute and then cold water quenched to room temperature.

Several mechanical properties have been determined after 1 month natural ageing at room temperature (T4 temper) and in a T64 temper (1 month natural ageing followed by simulated paint-bake cycle of 2% pre-stain and 185°C@20 minutes) to assess the paint-bake response of the sheet products.

The mechanical properties (yield strength Rp0.2, tensile strength Rm, uniform elongation Ag, elongation at fracture A80, strain hardening exponent n, r90°-value) have been assessed in accordance with international standard ISO 6892-1 (second edition, July 2016), and in Table 2, the average over three measurements per sample are listed.

Further, the Erichsen Dome Height (EDH) has been measured in accordance with EN ISO 20482 (July 2003). The EDH is used to assess the stretch formability of the sheet products in terms in plane stress biaxial tensile deformation. In Table 2, the average over three measurements per sample are listed. Table 1. Alloy composition of the aluminium alloys cast, the balance is made by aluminium and unavoidable impurities.

Table 2. Mechanical properties in T4 and T64 temper.

From the results of Table 2, it can be seen that an increased Fe-level (alloy 1 vs. alloy 2 and alloy 4 vs. alloy 5) increases the strength of the sheet product in T4 temper. Also, an increase in r90°-value and elongation and a decrease in n-value can be observed. These effects are believed to be due to the change of the grain size as observed by the addition of Fe. From the comparison, an increase in EDH can also be observed. The purposive addition of Sr (e.g., alloy 2 vs. alloy 3 and alloy 5 vs. alloy 6) further increases the strength in T4 temper, in particular, the ultimate tensile strength. In particular, an further increase in EDH can also be observed (e.g., alloy 2 vs. alloy 3 and alloy 5 vs. alloy 6). This improvement in formability, in particular in stretch formability assessed in an EDH test, at increasing Fe levels shows that the rolled 6xxx-series aluminium alloy sheet products in accordance with the invention are ideal candidates for forming into complex automotive members, in particular when applying forming tech- niques requiring better stretch formability. The capacity for scrap absorbing ensures a more economical effective and environmentally friendly production of such shaped complex automotive members.

EXAMPLE 2.

On an industrial scale of production, two rolling ingots having a thickness of about 455 mm after scalping have been produced. The alloy compositions are listed in Table 3, and whereby alloy no. 8 is an alloy according to the invention and has an Fe/Sr ratio of 10.

Each ingot has been homogenised for 9 hours at about 560°C and hot rolled to 7.5 mm. The hot-mill entry temperature was about 490°C. Next, the sheet products were cold rolled to 3 mm, followed by a batch interannealing at 380°C@2 hrs, and then cold rolled to a final gauge of 1 .0 mm. At final gauge, the sheet products have been solution heat-treated at 565°C and then cold water quenched to room temperature.

Table 3. Alloy composition of the aluminium alloys cast, the balance is made by aluminium and unavoidable impurities.

Several mechanical properties have been determined after 3 months natural ageing at room temperature (T4 temper) and in a T64 temper (3 months natural ageing followed by simulated paint-bake cycle of 2% pre-stain and 185°C@20 minutes) to assess the paint-bake response of the sheet products. The mechanical properties (yield strength RpO.2, tensile strength Rm, uniform elongation Ag, elongation at fracture A80) have been assessed in accordance with international standard ISO 6892-1 (second edition, July 2016), and in Table 4, the average over three measurements per sample are listed.

The Erichsen Dome Height (EDH) has also been measured in accordance with

EN ISO 20482 (July 2003). The EDH is used to assess the stretch formability of the sheet products in terms in plane stress biaxial tensile deformation. In Table 4, the average over three measurements per sample are listed. Table 4. Mechanical properties in T4 and T64 temper.

From the results of Table 4, it can be seen that the purposive addition of Sr results in an improved strength both in T4 and T64 condition. The formability by reference to the EDH results is also improved.

Samples of alloy 7 and 8 in the T4 condition have been analysed for several microstructural features, in particular the average grain size using standard light optical microscopy techniques at a magnification of 100x. The particle distribution of coarse Si particles larger than 0.35μηη has been analysed using standard light opti- cal microscopy techniques at a magnification of 500x, and the results are listed in Table 5. Further, the Mg2Si particles distribution larger than 0.35μηη has been analysed using SEM at a magnification of 500x, and the results are listed in Table 6.

The average grain size in the RD direction through thickness for alloy 7 was 58.2 m and for alloy 8 it was 50.2μηη. The average grain size in the ND direction through thickness for alloy 7 was 58.2 m and for alloy 8 it was 39.5μηη. Table 5. Microstructural feature: the coarse Si particles (>0.35μηη) distribution

Table 6. Microstructural feature: the Mg2Si (>0.35μηη) particles distribution

From these results, it can be seen that the purposive addition of Sr decreases the area fraction, the number density, and the size (circular equivalent average radius) of coarse Si particles, and also the area fraction, the number density and the size (circular equivalent average radius) of coarse Mg2Si particles. The addition of Sr decreases also the average grain size. This contributes to an increased formability of the 6xxx-series aluminium sheet material, in particular the stretch formability as indicated, for example, by the Erichsen Dome Height test results and increases also the elongation.

The invention is not limited to the embodiments described before, which may be varied widely within the scope of the invention as defined by the appending claims.

Claims

Claims

A rolled 6xxx-series aluminium alloy sheet product consisting of, in wt.%:

Mg 0.10% to 1 .3%,

Si 0.25% to 1 .4%,

Cu up to 0.45%,

Mn up to 0.50%,

Fe 0.10% to 0.45%,

Sr 0.01 % to 0.05%,

and wherein the ratio Fe/Sr is in a range of 4 to 40,

Ti up to 0.20%,

Cr up to 0.30%,

Zr up to 0.20%,

V up to 0.20%,

Zn up to 0.35%,

Sn up to 0.075%,

other elements and impurities each <0.05%, total <0.15%, balance aluminium. The rolled 6xxx-series aluminium alloy sheet product according to claim 1 , wherein the Si-content is at least 0.50%, and preferably at least 0.65%.

The rolled 6xxx-series aluminium alloy sheet product according to claim 1 or 2, wherein the Mg-content is less than 1 .3%, and preferably less than 1 .0%.

The rolled 6xxx-series aluminium alloy sheet product according to any one of claims 1 to 3, wherein the ratio Si/Mg is at least 0.90.

The rolled 6xxx-series aluminium alloy sheet product according to any one of claims 1 to 4, wherein the ratio Si/Mg is in a range of 0.90 to 1 .40. The rolled 6xxx-series aluminium alloy sheet product according to any one of claims 1 to 4, wherein the ratio of Si/Mg is in a range of 2.0 to 7.0, and preferably 2.5 to 7.0.

The rolled 6xxx-series aluminium alloy sheet product according to any one of claims 1 to 6, wherein the Fe/Sr ratio is in a range of 4 to 40, and preferably 5 to 40, and preferably 6 to 40, and more preferably 6 to 20.

8. The rolled 6xxx-series aluminium alloy sheet product according to any one of claims 1 to 7, wherein the Mn-content is in a range of 0.02% to 0.50%, and preferably in a range of 0.02% to 0.20%.

9. The rolled 6xxx-series aluminium alloy sheet product according to any one of claims 1 to 8, wherein the Cr-content is in a range of 0.01 % to 0.30%, and preferably in a range of 0.01 % to 0.20%.

10. The rolled 6xxx-series aluminium alloy sheet product according to any one of claims 1 to 9, wherein the Cu-content is at least 0.02%, and preferably at least 0.04%.

1 1 . The rolled 6xxx-series aluminium alloy sheet product according to any one of claims 1 to 10, wherein the Cu-content at most 0.40%, and preferably at most 0.20%. 12. The rolled 6xxx-series aluminium alloy sheet product according to any one of claims 1 to 1 1 , wherein the Fe-content at least 0.18%, and preferably at least 0.20%, and more preferably at least 0.22%.

13. The rolled 6xxx-series aluminium alloy sheet product according to any one of claims 1 to 12, wherein the aluminium alloy sheet product has a gauge in the range of 0.7 mm to 4.0 mm.

14. The rolled 6xxx-series aluminium alloy sheet product according to any one of claims 1 to 13, wherein said sheet product is provided in the form of a shaped three-dimensional automotive body member.

15. Method of manufacturing a rolled 6xxx-series aluminium alloy sheet product according to any one of claims 1 to 14, wherein the method comprises the steps of:

(a) semi-continuously or continuously casting an ingot of an aluminium alloy according to any one of claims 1 to 12;

(b) homogenising of the ingot;

(c) rolling of the ingot into an aluminium alloy rolled sheet product having a final gauge in a range of 0.7 mm to 4.0 mm;

(d) solution heat treating the aluminium alloy sheet product at a temperature of at least 500°C;

(e) after the solution heat treating, quenching of the aluminium alloy sheet product; and

(f) natural ageing of the quenched aluminium alloy sheet product.

16. A method according to claim 15, further comprising artificially ageing of the solution heat treated and quenched rolled sheet product.

17. A method of manufacturing a rolled 6xxx-series aluminium alloy sheet product according to any one of claims 15 or 16, the method comprising:

continuously casting the aluminium alloy according to any one of claims 1 to 12;

rolling the aluminium alloy into an aluminium alloy rolled sheet product having a final gauge in the range of 0.7 to 4.0 mm;

solution heat treating the aluminium alloy sheet product at a temperature of at least 500°C;

after the solution heat treating, quenching of the aluminium alloy sheet product; and

natural ageing of the quenched aluminium alloy sheet product.

18. A method of manufacturing a rolled 6xxx-series aluminium alloy sheet product according to any one of claims 15 to 17, wherein solution heat treating the aluminium alloy sheet product is at a temperature in a range of 520°C to 570°C.

19. A method of manufacturing a rolled 6xxx-series aluminium alloy sheet product according to any one of claims 15 to 18, wherein the solution heat treated and quenched aluminium alloy sheet product has been pre-aged, preferably pre-aged at a temperature in a range of 50°C to 130°C.

20. A method of manufacturing a rolled 6xxx-series aluminium alloy sheet product according to any one of claims 15 to 19, wherein the natural ageing of the quenched aluminium alloy sheet product if for at least 72 hours.

21 . A method of manufacturing a rolled 6xxx-series aluminium alloy sheet product according to any one of claims 15 to 20, wherein the method further comprising the step of shaping of the 6xxx-series aluminium alloy sheet product into a three-dimensional form.

22. A method of manufacturing a rolled 6xxx-series aluminium alloy sheet product according to claim 21 , wherein the 6xxx-series aluminium alloy sheet product in a three-dimensional form is subjected to a paint-bake cycle.

23. Use of a rolled 6xxx-series aluminium alloy sheet product according to any one of claims 1 to 13 or obtained by the method according to any one of claims 15 to 22 form as a shaped three-dimensional automotive body member.