PRODUCT FIELD OF THE INVENTION

The invention relates to a method of manufacturing an Al-Mg alloy rolled sheet product for very good formability characteristics. The sheet product can be used in a wide range of applications, and ideally as automotive body sheet. BACKGROUND OF THE INVENTION

It is known that the Al-Mg series aluminium alloy exhibit a fairly good balance of strength, corrosion resistance and formability. Al-Mg alloys are also very good weldable. The representative material for these Al-Mg series alloys is the AA5182 alloy, but includes also AA5082, and AA5086.

The chemical composition (in wt.%) of AA5082, AA5182, and AA5086 are listed in the

Table 1 below, with others each <0.05%, total <0.15%, and balance aluminium.

Table 1.

International patent application WO-2014/029856-A1 discloses a cold-rolled alumin ium alloy strip of an AIMg alloy having a composition, in wt.%, Mg 4.1 %-4.5%, Mn 0.2%- 0.35%, Si up to 0.2%, Fe up to 0.35%, Cu up to 0.15%, Cr up to 0.1 %, Zn up to 0.25%, Ti up to 0.1 %, balance aluminium and impurities, and wherein the aluminium alloy strip has a recrystallized microstructure, the grain size of the microstructure ranges from 15pm to 25pm and the final soft annealing of the aluminium alloy strip is performed in a continuous furnace. The disclosed method of manufacturing such as AIMg alloy strip comprises the steps of casting a rolling ingot, homogenisation of the ingot at 480°C to 550°C for at least 0.5 hours, hot rolling of the rolling ingot at a temperature of 280°C to 500°C, cold rolling of the alumin ium strip to a final thickness with a degree of rolling of 40% to 70% or 50% to 60%, optionally intermediate annealing at 300°C to 500°C during cold rolling, and soft annealing of the fin ished-rolled aluminium alloy strip at 300°C to 500°C in a continuous furnace. The AIMg sheet material is said to have improved resistance to intercrystalline corrosion and remains to have a good formability. The known AIMg sheet material can be used for motor vehicle body parts.

US patent document US-2015/0159250-A1 discloses a method for producing an AI Mg alloy strip having 4.1-4.5% Mg, 0.2-0.35% Mn, up to 0.2% Si, up to 0.35% Fe, up to 0.15% Cu, up to 0.1 % Cr, up to 0.25% Zn, up to 0.1 % Ti, balance aluminium and impurities, the method comprising the steps of casting a rolling ingot; homogenisation of the rolling ingot at 480-550°C for at least 0.5 hours; hot rolling of the rolling ingot at a temperature of 280-500°C; cold rolling of the aluminium strip to the final thickness with a degree of rolling of 40-70% or 50-60%; soft annealing of the finished-rolled aluminium alloy strip at 300- 500°C in a continuous furnace.

US patent document US-4, 151 ,013 discloses a method of manufacturing Al-Mg sheet products comprising 4-5% Mg and 0.20-0.50% Mn and without being marked by Type-A Luders lines, the method comprising the steps of cold rolling of the aluminium sheet; heating the sheet to a temperature in the range of 850°F to 1050°F; quenching the sheet down to about 350°F at a defined quench rate; and then uniformly stretching the sheet to effect a permanent set along its length of from about 0.25% to about 1 %, preferably of about 0.5%.

There is a demand for Al-Mg sheet material having further improved formability char acteristics.

It is an object of the invention to provide a method of manufacturing an Al-Mg alumin ium alloy sheet product having a balance of good formability and high strength.

DESCRIPTION OF THE INVENTION

As will be appreciated herein below, except as otherwise indicated, aluminium alloy and temper designations refer to the Aluminium Association designations in Aluminum Standards and Data and the Registration Records, as published by the Aluminium Associ ation in 2018 and are well known to the persons skilled in the art.

For any description of alloy compositions or preferred alloy compositions, all refer ences 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 0.10% Zn may include an alloy having no Zn.

This and other objects and further advantages are met or exceeded by the present invention providing a method of manufacturing an Al-Mg or 5xxx-series aluminium alloy sheet product, the method comprising the steps of:

(a) providing a rolling feedstock material of an aluminium alloy having a composi tion comprising of, in wt.%,

Mg 3.5% to 5.25%,

Mn 0.2% to 0.8%,

Fe up to 0.40%,

Si up to 0.30%,

Cu up to 0.15%,

Cr up to 0.25%,

Zr up to 0.25%,

Zn up to 0.60%,

Ti up to 0.1 %,

unavoidable impurities, typically each up to 0.05% and total up to 0.15%, and the balance aluminium;

(b) preheating and/or homogenisation of the rolling feedstock, preferably at a tem perature of 480°C to 550°C for at least 0.5 hours;

(c) hot rolling of the rolling feedstock, typically at a temperature of 270°C to 540°C;

(d) cold rolling to a final gauge with a cold rolling reduction in a range of 25% to

85%;

(e) annealing of the cold rolled sheet material at final gauge by two separate or discrete annealing treatments with a first annealing step at a temperature between 100°C and 300°C followed by a second annealing step at a temperature between 470°C and 540°C;

(f) and optionally followed by a stretching operation, preferably at a leveller, with a maximum elongation of 0.7%, preferably of maximum 0.5%, of the annealed sheet material. Thereafter the sheet material is coiled and stored for shipment. In accordance with the invention it has been found that process step (e) is a key process parameter and results in a less critical process operating window, in particular for the final annealing heat-treatment, for manufacturing a rolled aluminium alloy sheet product having an improved balance in strength and formability characteristics. The rolled aluminium alloy sheet product has also a good corrosion resistance, in particular against intergranular corrosion.

The cold-rolled and annealed aluminium sheet product manufactured in accordance with the invention has a fully recrystallized microstructure, and preferably the average grain size of the microstructure ranges from 8 pm to 50 pm, and preferably from 9 pm to 25 pm. In this context grain size is taken to mean the average grain diameter according to ASTM- E-112, on the cross-section rolling direction x normal direction. This achieves amongst others the effect that the formability of the aluminium sheet is very good, while the formation of so-called type-A Luders lines is being suppressed. If the grain size in the aluminium sheet is too large, then a so-called orange peel can form during pressing, which is an undesired effect. If the grain size is lower than 8 pm the YPE will increase to above 1.5% which corresponds to a poor formability. Smaller grain size makes to material also more prone to IGC.

The cold rolled and annealed aluminium sheet product manufactured in accordance with the invention provides a balance of relevant engineering properties, in particular: a 0.2% offset yield strength (YS) of more than 110 MPa,

an ultimate tensile strength (UTS) of more than 255 MPa, and preferably more than 260MPa,

an elongation at fracture (A) of more than 22%,

an r-value (rgo) (at 10% strain) of more than 0.60,

a yield point elongation (A _e or YPE) of less than 0.60%.

The properties are to be measured using samples per ISO-6892 Type-I in the transverse rolling direction of the aluminium sheet.

The Al-Mg aluminium alloy 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. DC-casting, EMC-casting, EMS-casting, and preferably having an ingot thickness in a range of about 300 mm or more, e.g. 400 mm, 500 mm or 600 mm. In another embodiment thinner gauge slabs resulting from continuous casting, e.g. belt casters or roll casters, also may be used to provide Al-Mg rolling feedstock, and having a thickness of up to about 40 m .

After casting the rolling feedstock, in particular the thick as-cast ingot is commonly scalped to remove segregation zones near the cast surface of the cast ingot.

The aluminium alloy rolling stock is preferably preheated and/or homogenized at a temperature of at least 480°C prior to hot rolling in single or multiple rolling steps. To avoid eutectic melting resulting in possible undesirable pore formation within the ingot the tem perature should not be too high and should typically not exceed 550°C, and preferably not 535°C. The time at temperature for a large commercial size ingot should be at least 0.5 hours and can be about 1 to 36 hours. A longer period, for example 48 hours or more, has no immediate adverse effect on the desired properties, but is economically unattractive.

In a next step the aluminium alloy rolling stock is hot rolled at 270°C to 550°C, prefer ably 290°C to 520°C, to an intermediate gauge typically of about 3 mm to 8 mm, followed by cold rolling to final gauge.

The cold rolling step reduction is important to arrive at the final gauge. The cold rolling degree before the final annealing is one of the key parameters in AIMg- or 5xxx-series alu minium alloys to arrive at the required grain size in the final product after annealing. The higher the cold rolling reduction, the lower the average grain size in the final sheet product. However, when the grain size is excessively large, ductility and formability deteriorate. The cold rolling to a final gauge in the method according to the invention is by a cold rolling reduction before final annealing in a range of 25% to 85% to ensure recrystallization throughout the aluminium alloy sheet during annealing.

An intermediate annealing at a temperature in the range of 300°C to 530°C during cold rolling can be performed in either a batch furnace or a continuous annealing furnace as is known in the art and has no adverse effect on the formability of the final sheet product.

Following the cold rolling to final gauge, the rolled sheet material is annealed in ac cordance with the invention by two separate or discrete annealing treatments in a first an nealing step at low temperature followed by a second annealing step at a high temperature.

The first annealing step is at a temperature in the range of 100°C to 300°C. A pre ferred lower-limit for the first annealing temperature is 140°C, and more preferably 150°C. A preferred upper-limit for the first annealing temperature is 250°C, more preferably 220°C, and more preferably 200°C. The time at the annealing temperature is for 1 second to 300 seconds, and preferably for 2 seconds to 120 seconds, and more preferably 5 seconds to 120 seconds. During the first annealing step only recovery of the microstructure takes place in the aluminium sheet and no recrystallisation.

It is assumed that due to the first annealing step the amount of stored energy is low ered and which changes the nuclei selection during subsequent recrystallization in the sub sequent second annealing step as the nucleation of grains other than cube-grains is de layed to higher temperature and thereby creating a texture which is more stable during normal grain growth. This in turn results in a less critical process operating window, in par ticular for the second annealing at a temperature between 470°C and 540°C, by which an YPE of less than 0.60% can be obtained in combination with a desirable r-value of more than 0.60 in the resultant aluminium alloy sheet material.

The second annealing step is at a temperature in the range of 470°C to 540°C and is preferably performed in a continuous annealing furnace is to provide a fully recrystallized microstructure in the aluminium sheet product. Continuous annealing requires a rapid heat up of the moving aluminium sheet to the annealing temperature. The average heat-up rate is more than 5°C/sec, and preferably more than 10°C/sec. A preferred lower-limit for the annealing temperature is about 490°C, and more preferably >500°C. A preferred upper-limit is about 535°C, and more preferably about 530°C. The time at the annealing temperature is for 1 second to 300 seconds, and preferably for 5 seconds to 60 seconds, and more preferably for 10 seconds to 60 seconds. Annealing in a continuous annealing furnace is favoured annealing in a batch furnace requiring much longer heat-up and soaking times. For the same recrystallized grain size continuous annealing has the advantage to signifi cantly reduce the formation of so-called stretcher strain markings in comparison to batch annealing. Immediately following this second annealing step the aluminium sheet is cooled preferably to below about 150°C, more preferably to below about 100°C, and most prefer ably to about ambient temperature, using a cooling rate of at least 25°C/sec and then coiled.

The first annealing step can be carried out as a separate heat-treatment whereby preferably a coil of the cold rolled feedstock is uncoiled and heated and soaked at the de fined temperature and time, followed by re-coiling. Following the soaking at the defined temperature and time the feedstock is preferably cooled by forced air cooling or water quenching prior to re-coiling. The first step annealed and re-coiled feedstock is stored until the second annealing step is being performed.

The heating of the uncoiled aluminium sheet material for performing the first annealing step can be done in various ways, in particular the heating is selected from the group con sisting of infrared, radiant-tube, gas-fired furnace, direct resistance, induction heating, and combinations thereof. In an alternative embodiment the first and second annealing step are positioned and performed in-line such that uncoiled feedstock having been subjected to the first annealing step is directly, without any re-coiling, fed into a continuous annealing furnace for performing the second annealing step as herein described and claimed.

In a preferred embodiment, the second annealing step is followed by an operation to improve product flatness. The usual methods for straightening can be applied and include skin-pass rolling with a light reduction rate, levelling with bending and unbending by passing through the straightening rolls, if necessary, further together with applying a tension, and stretching to impart a low tensional deformation. More preferably a stretching operation at a leveller device is performed with a maximum elongation of 0.7%, preferably of maximum 0.5%, of the annealed sheet material to increase sheet product flatness. The stretching operation is preferably performed at ambient temperature. Thereafter the levelled sheet material is coiled and stored for shipment.

In an embodiment the aluminium alloy sheet product after the last cold rolling step has a gauge in a range of about 0.5 mm to 4 mm. A preferred lower-limit for the gauge is about 0.7 mm. A preferred upper-limit for the thickness is about 3 mm, and more preferably about 2.5 mm.

In the aluminium alloy sheet product manufactured in accordance with the method of the invention the Mg-content should be in a range of 3.5% to 5.25% and forms the primary strengthening element of the aluminium alloy. A preferred upper-limit for the Mg content is 5.0%.

The Mn-content should be in the range of about 0.20% to about 0.8% and is another essential alloying element. In a preferred embodiment the Mn-content is in a range of about 0.20% to about 0.5%.

In one embodiment the aluminium alloy has Mg-content in the range of 4.1 % to 4.5%, preferably 4.2% to 4.4%, preferably in combination with a Mn-content of 0.3% to 0.5%, preferably 0.3% to 0.45%.

In another embodiment the aluminium alloy has Mg-content in the range of 4.5% to 5.25%, preferably 4.5% to 5.0%, preferably in combination with a Mn-content of 0.2% to 0.35%, preferably 0.2% to 0.3%.

To control the microstructure of the final product, next to the addition of Mn, it is pre ferred to have a purposive addition of either Cr or Zr each up to about 0.25% as dispersoid- forming elements. A preferred addition of Cr is in a range of up to 0.05%. When Cr is added purposively then it is preferred that the Zr level does not exceed 0.02%, and is preferably less than about 0.01 %.

Iron (Fe) is a common impurity and can be present in a range of up to about 0.40% and preferably is kept to a maximum of about 0.35%, and is preferably more than 0.10%. Al-Mg sheet material processed in accordance with the invention provides very good form- ability characteristics without having to lower the Fe-content to very low levels (i.e. less than 0.25%, and typically in a range of 0.15% to 0.25%), although lowering the Fe-content to low levels would further enhance the formability characteristics.

Silicon (Si) is a common impurity and can be present in a range of up to about 0.30% and preferably is kept to a maximum of about 0.2%. A typical preferred Si level would be in the range of up to 0.10%.

As the corrosion resistance remains an important engineering property in the sheet material, it is preferred to maintain the copper (Cu) at a low level of 0.15% or less.

Zinc (Zn) can be present up to about 0.60% to increase the strength of the aluminium alloy sheet. In applications of the aluminium sheet material where corrosion resistance is an important engineering parameter then Zn should be treated as a common impurity and can be present in a range of up to about 0.25%, and preferably is kept to a maximum of about 0.15%, and more preferably at a maximum of about 0.10%, as it may have in partic ular an adverse effect on the corrosion resistance test results, in particular NAMLT test results according to ASTM G67-13.

Ti is important as a grain refiner during solidification of both ingots and welded joints produced using the alloy sheet product of the invention. Ti levels should not exceed about 0.1 %, and the preferred range for Ti is about 0.005% to 0.05%. Ti can be added as a sole element or as is known in the art with either boron or carbon serving as a casting aid for grain size control.

In an embodiment of the invention the Al-Mg aluminium alloy consists of, in wt.%: Mg 3.5% to 5.25%, Mn 0.2% to 0.8%, Fe up to 0.40%, Si up to 0.30%, Cu up to 0.15%, Cr up to 0.25%, Zr up to 0.25%, Zn up to 0.60%, Ti up to 0.1 %, unavoidable impurities each <0.05%, total <0.15%, balance aluminium; and with preferred narrower compositional ranges as herein described and claimed.

Fig. 1 shows a schematic illustration of the processing steps of an exemplary embod iment for producing an Al-Mg sheet product based on the present invention. In process step A an rolling ingot in cast having a composition of an Al-Mg alloy as herein described and claimed. In process step B the rolling ingot is homogenised and in step C hot rolled to an intermediate gauge. Next in process step D the hot rolled material is cold rolled to final gauge. Optionally an intermediate annealing is applied in process step E during the cold rolling operation. The cold rolled material at final gauge is annealed in a first annealing step F followed by a distinct and separate second annealing at higher temperature in process step G. The cold-rolled and annealing aluminium sheet may be stretched in process step H and subsequently coiled.

In a further aspect of the invention it relates to a cold-rolled and annealed aluminium sheet product produced using the method according to this invention and wherein the aluminium sheet product has in the transverse rolling direction:

a 0.2% offset yield strength (YS) of more than 110 MPa,

an ultimate tensile strength (UTS) of more than 255 MPa, and preferably more than 260MPa,

an elongation at fracture (A) of more than 22%,

an r-value (rgo) (at 10% strain) of more than 0.60, and

a yield point elongation (A _e or YPE) of less than 0.60%.

In a further aspect of the invention it relates to the use of an aluminium alloy sheet product obtained by the method according to this invention as an automotive sheet product for manufacturing motor vehicle body parts, in particular for an inner door part, an inner tailgate part, and Body-in-White (BIW) parts, often requiring complexly formed geometries, so that the good forming behaviour obtained by this invention constitutes a very important improvement for providing the complex geometries. However, the aluminium alloy sheet product can be used also in electronics, industrial and other applications.

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

EXAMPLE

On an industrial scale of manufacturing rolling ingots have been DC-cast of an AA5182-series Al-Mg alloy having the following composition, 4.38% Mg, 0.36% Mn, 0.28% Fe, 0.14% Si, 0.02% Cr, 0.01 % Cu, 0.01 % Zn, 0.02% Ti, total impurities <0.03%, balance aluminium. The rolling ingots have been homogenised for about 10 hours at a temperature of 535°C. Next hot rolled to a final gauge of 5 mm using a hot-mill entry-temperature of about 510°C and a hot-mill exit-temperature of about 335°C. In multiple cold rolling steps the hot rolled material has been cold rolled to two different final gauges, namely of 2.84 mm (cold rolling reduction about 43%) and 1.56 mm (cold rolling reduction about 69%).

The cold rolled sheet material at final gauges has been annealed in a continuous anneal furnace at several temperatures (410°C, 450°C, 510°C, 530°C, and 560°C) using a soaking time of about 15 seconds and quenched. And one series of cold rolled sheet ma terial has been annealed in accordance with the invention by applying a first annealing at a temperature of about 165°C for about 6 seconds prior to the annealing in a continuous annealing furnace at several temperatures of 410°C, 450°C, 510°C, 530°C, and 560°C. The final annealed sheet products have been levelled to improve product flatness and resulting in a sheet elongation of about 0.1 % maximum.

For all sheet products the YPE (%), yield strength (YS) in MPa, ultimate tensile strength (UTS) in MPa, elongation at fracture (A) in %, and the r-value have been measured in the traverse rolling direction. The results (average over three samples) are listed in Table 1 and Table 2 below for the 2.84 mm and 1.56 mm sheet material respectively; means not measured. Also for each sheet material the average grain size (in micron) has been measured. In all cases the aluminium sheets had a fully recrystallized microstructure.

Table 1. Mechanical properties for the 2.84 mm sheet material as function of the ap plied annealing practices.

From the results of Table 1 it can been seen amongst others that with increasing second annealing temperature the YPE, YS, UTS, and r-value decrease.

A too low second annealing temperature of 410°C and 450°C, both with and without a first annealing, provides very high mechanical strength but in combination with an unac ceptable high YPE of more than 0.60.

A too high second annealing temperature of about 560°C, both with and without a first annealing, provides a too low YS of less than 110 MPa. And in the case of no first annealing also a too low elongation at fracture.

In particular the balance between YPE and YS is important for providing a good form- ability in combination with an r-value of more than 0.60; the YPE should be less than 0.60 and the YS should be more than 1 10 MPa.

In accordance with this invention it has been found that by applying the second an nealing at a temperature in a range of 470°C to 540°C, and preferably in a range of 490°C to 540°C, and more preferably >500°C-540°C, in combination with a first annealing temper ature in a range of 100°C to 300°C, a desired balance in the relevant properties can be achieved. The first annealing at lower temperatures provides a much broader operating temperature range for the second annealing temperature while providing the relevant bal ance in properties in the aluminium sheet. Without the first annealing the operating window is very narrow, and in many cases the required set of properties are not achieved. A broader and thus less critical second annealing temperature range is favourable in an industrial en vironment of producing aluminium sheet products; as otherwise a small temperature fluctu ation might lead to sheet products not, or only marginally, meeting customer requirements on various essential engineering properties.

Table 2. Mechanical properties for the 1.56 mm sheet material as function of the ap plied annealing practices.

Similar as for Table 1 , From Table 2 it can been seen amongst others that with in- creasing second annealing temperature the YPE, YS, UTS, and r-value decrease.

A too low second annealing temperature of 410°C and 450°C, both with and without a first annealing, provides desirable very high mechanical strength but in combination with an unacceptable high YPE of more than 0.60.

A too high second annealing temperature of about 560°C, both with and without a first annealing, provides a too low YS of less than 110 MPa. And in the case of no first annealing also a too low elongation at fracture.

In particular the balance between YPE and YS is important for providing a good form- ability in combination with an r-value of more than 0.60; the YPE should be less than 0.60 and the YS should be more than 110 MPa.

In accordance with this invention it has been found that by applying the second an nealing at a temperature in a range of 470°C to 540°C, and preferred narrow ranges, in combination with a first annealing temperature in a range of 100°C to 300°C, a desired balance in the relevant properties can be achieved. The first annealing at lower tempera tures provides a broader operating temperature range for the second annealing temperature while providing the relevant balance in properties in the aluminium sheet. Without the first annealing the operating window is very narrow, and in many cases the required set of prop erties are not achieved at all. For example a comparison of the results of sheet 13 and 14 suggest that the required set of properties might be achievable when applying only a second annealing at a temperature somewhere between 510°C and 530°C, but which would require additional tests and would result only in a very narrow temperature operating window, if successful at all. A broader and thus less critical second annealing temperature range is favourable in an industrial environment of producing aluminium sheet products; as other wise a small temperature fluctuation might lead to Al-Mg sheet products not, or only mar ginally, meeting customer requirements on various essential engineering properties.

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