Patent Application
20150159250
The invention relates to a cold-rolled aluminium alloy strip composed of an AlMg aluminium alloy and a method for the production thereof. Furthermore, corresponding components produced from the aluminium alloy strips will be proposed.
Aluminium-magnesium-(AlMg-)-alloys of the AA 5xxx type are used in the form of sheets or plates or strips for the construction of welded or joined structures in ship, automotive and aircraft construction. They are characterised by high strength which increases as the magnesium content rises. AlMg-alloys of the AA 5xxx type with Mg contents of more than 3%, in particular more than 4%, have an increasing tendency towards intercrystalline corrosion, when exposed to high temperatures. At temperatures of 70-200° C., β-AlsMg3 phases precipitate along the grain boundaries, which are referred to as β-particles and in the presence of a corrosive medium can be selectively dissolved. The result of this is that the AA 5182-type aluminium alloy (Al 4.5% Mg 0.4% Mn) having very good strength properties and very good formability in particular cannot be used in heat-stressed areas, where the presence of a corrosive medium such as water in the form of moisture must be contended with. This concerns in particular the components of a motor vehicle which normally undergo cathode dip painting (CDP) and are then dried in a stoving process, as already due to this stoving process, normal aluminium alloy strips can become susceptible to intercrystalline corrosion. Furthermore, for use in the automotive sector, forming during the manufacture of a component and subsequent operational stressing of the component must be taken into consideration.
The susceptibility to intercrystalline corrosion is normally checked in a standard test (NAMLT test) according to ASTM G67, during which the specimens are exposed to nitric acid and the mass loss due to the intercrystalline corrosion is measured. According to ASTM G67, the mass loss of materials which are not resistant to intercrystalline corrosion, is more than 15 mg/cm2.
Sheet metal for the automotive industry, e.g. for internal door parts, must have very good formability. Here, the requirements are essentially determined by the stiffness of the component concerned, with the strength of the material playing only a subordinate role. The components often undergo multi-stage forming processes, such as for example doors with integrated window frame areas.
So, apart from the corrosion properties, the formability of the AlMg aluminium alloy also has a major influence on the usage possibilities for this material. For example, the materials known so far have meant that it is not possible for the side walls of a motor vehicle to be deep-drawn from a single sheet, making not only reconstruction of the side wall but also additional process steps for providing the side wall of a motor vehicle necessary.
The forming behaviour can, for example, be measured in a stretch drawing trial by an Erichsen cupping test (DIN EN ISO 20482), in which a test piece is pushed against the sheet, resulting in cold forming. During the cold forming, the force and the force displacement of the test piece are measured, until a load drop occurs, caused by the formation of a crack. The SZ32 stretch drawing measurements quoted in the application were performed with a punch head diameter of 32 mm and a die diameter of 35.4 mm with the help of a Teflon deep-drawing film to reduce friction. Further measurements of the deep drawability were performed using the so-called plane-strain-cupping test using a Nakajima geometry according to DIN EN ISO 12004 with a punch diameter of 100 mm. For this, specimens with a specific geometry underwent drawing tests until a crack appeared, with the depth until cracking being used as a measure of the formability of the material.
From JP 2011-052290 A, an aluminium alloy strip for can lids is known, which is preferably load-resistant despite its small thickness. Here, the strip has a recrystallized microstructure.
Further, from EP 2 302 087 A1, a chassis part is known made from an aluminium composite material, which has aluminium alloy layers as outer layers. Due to the alloying constituents, the Al composite material is characterized by excellent strength values with a high corrosion resistance at low weight.
Composite material solutions composed of AA5xxx aluminium alloys with a high Mg content and outer aluminium alloy layers to protect against corrosion, however, have the disadvantages that manufacture is complex and additionally at joining points where the aluminium composite material joins to other parts, for example at cutting edges, drill-holes and breakouts, there is furthermore an increased danger of corrosion.
The present invention is therefore concerned with single-layer aluminium materials. On this basis, the object of the invention is to provide a single-layer aluminium alloy strip, having sufficient resistance to intercrystalline corrosion but nevertheless having good formability, so that large-area, deep-drawn parts, such as interior parts of motor vehicles doors, with sufficient strength can be provided. Furthermore, a method will be indicated with which single-layer aluminium alloy strips can be produced. Finally, components produced from the aluminium alloy strips according to the invention will be indicated.
According to a first teaching of the present invention, the object indicated is achieved by a cold-rolled aluminium alloy strip composed of an AlMg aluminium alloy, wherein the aluminium alloy has the following alloying elements:
It has been found that within the specification of the AA5182-type aluminium alloy, there is a specific, narrow, alloying range which offers sufficient resistance to intercrystalline corrosion and at the same time, by taking into account certain constraints, such as for example the average grain size and the type of final soft annealing, results in an exceptional forming behaviour. In particular, the combination of the average grain size with the claimed alloying elements of the aluminium alloy of the aluminium alloy strip means that degrees of formability can be achieved allowing the production of large-area design, deep-drawn sheet aluminium products with sufficient strength. In particular it has been found that the use of a continuous furnace rather than the normal coil annealing performed in a chamber furnace provides a further significant increase in formability.
According to a first configuration of the aluminium alloy strip, the aluminium alloy also has one or more of the following restrictions to the contents of alloying elements:
Restricting the alloying content for copper to a maximum of 0.1 wt. % leads to an improvement in the corrosion resistance of the aluminium alloy strip. A Cu content of 0.04 wt. % to 0.08 wt. % ensures that the copper contributes to an increase in strength, but that nevertheless the corrosion resistance is not reduced too sharply. Silicon, chromium, zinc and titanium contents higher than the values indicated lead to a worsening of the formability of the aluminium alloy. The amount of silicon present in the alloy of 0.03 to 0.1 wt. %, in combination with the iron and manganese components in the stated quantities, in particular leads to relatively evenly distributed, compact particles of the quaternary α-Al(Fe,Mn)Si-phase, increasing the strength of the aluminium alloy, without negatively influencing other properties such as the formability or corrosion behaviour.
Titanium is normally added during continuous casting of the aluminium alloy as a grain refiner, for example in the form of titanium boride wire or rods. Therefore in a further embodiment the aluminium alloy has a Ti content of at least 0.01 wt. %.
A further improvement in the corrosion behaviour and the formability of the aluminium alloy strip can be achieved by the aluminium alloy also having one or more of the following restrictions to the contents of alloying elements:
It has been found that chromium in contents below the contamination threshold of 0.05 wt. % significantly influences the formability of the aluminium alloy strip and therefore should be contained in the smallest possible proportions in the aluminium alloy of the aluminium alloy strip according to the invention. The zinc content is set at below the contamination threshold of 0.05 wt. %, in order not to impair the general corrosion behaviour of the aluminium alloy strip.
It has furthermore been found that iron within the values permitted according to the AA5182-type aluminium alloy in conjunction with silicon and manganese contents as described above has an effect on the formability. In combination with silicon and manganese, iron contributes to the thermal stability of the aluminium alloy strip, so that preferably the Fe-content of the aluminium alloy strip according to a next configuration is 0.1 wt. % to 0.25 wt. % or 0.10 wt. % to 0.20 wt. %.
The same also applies to the Mn content of a further configuration of the aluminium alloy strip, which should preferably be limited to 0.20 wt. % to 0.30 wt. %, in order to achieve optimum formability of the aluminium alloy strip.
An especially good compromise between the provision of high strength properties, good corrosion resistance to intercrystalline corrosion and improved forming properties can be achieved according to a further configuration of the aluminium alloy strip with an Mg content of 4.2 wt. % to 4.4 wt. %.
In order to provide the strength properties necessary for the areas of application, the aluminium alloy strip according to a next embodiment has a thickness of 0.5 mm to 4 mm. The thickness is preferably 1 mm to 2.5 mm, since most of the areas of application of the aluminium alloy strip fall within this range.
Finally, in the automotive sector the aluminium alloy strip according to the invention allows areas of application wherein the aluminium alloy strip in the soft state has a yield point Rp0.2 of at least 110 MPa and a tensile strength Rm of at least 255 MPa. It has been found that aluminium alloy strips with such yield points and tensile strengths especially are particularly well-suited for use in the automotive sector.
According to a second teaching of the present invention the object shown above is achieved by a method for producing an aluminium alloy strip according to the embodiments described above, wherein the method comprises the following process steps:
It has been found that with the indicated parameters in conjunction with the stated aluminium alloying components an aluminium alloy strip with average grain sizes of 15 μm-30 μm can be produced, having sufficient resistance to intercrystalline corrosion, providing sufficient strength properties and also having very good forming properties, so that large-area, deep-drawn sheet metal parts can be produced. Homogenisation of the rolling ingot ensures a homogenous structure and a homogenous distribution of the alloying elements in the hot rolling ingots to be rolled. The hot rolling at temperatures of 280° C.-500° C. allows recrystallization throughout during hot rolling, wherein the hot rolling typically is performed up to a thickness of 2.8 mm-8 mm. The final cold-rolling step is restricted to a degree of rolling of 40% to 70% or 50% to 60%, in both cases in order to ensure recrystallization throughout the aluminium alloy strip during soft annealing. The higher the degree of rolling of the aluminium alloy strip, the lower the average grain sizes become, wherein it has been found that above a 70% degree of rolling in the final soft annealing an average grain size can result that is too low. At a degree of rolling below 40% during soft annealing the average grain sizes are on the other hand too large, so that despite the resistance to intercrystalline corrosion increasing, the formability is nevertheless reduced. Soft annealing of the finish-rolled aluminium alloy strip takes place in a continuous furnace, which will normally have a heat-up rate of 1-10° C./second and so unlike a chamber furnace, in which an entire coil is heated, because of the rapid heating will have a marked effect on the later properties of the structure of the aluminium alloy strip. In particular, it has been possible to establish that during soft annealing in the continuous furnace an improved formability of the strip compared to variants annealed in the chamber furnace is achieved.
Alternatively, according to a further embodiment of the method, the aluminium alloy strip can also be produced with an intermediate annealing. According to this alternative variant after hot rolling alternatively the following process steps are performed:
The intermediate annealing of the aluminium alloy strip can take place both in the chamber furnace and in the continuous furnace. An effect on formability could not be determined. The decisive factors here are the degree of rolling achieved in cold rolling to the final thickness and if the soft annealing of the strip takes place in the continuous furnace. This determines the formability and corrosion resistance in conjunction with the alloying composition, irrespective of the type of intermediate annealing.
In order to prevent a further change in the microstructural state in the coiled condition following soft annealing, the aluminium alloy strip according to a further configuration of the method is cooled after soft annealing to a maximum temperature of 100° C., preferably a maximum of 70° C. and then coiled.
As already stated above, the intermediate annealing can be carried out in a further configuration of the method in a batch furnace or in a continuous furnace.
If the aluminium alloy strip is cold-rolled to a final thickness of 0.5 mm-4 mm, preferably to a final thickness of 1 mm-2.5 mm, this provides the typical areas of application, in particular automotive construction, with sheet metal with very good formability, and which can be deep-drawn with large surface areas and at the same time provide high strength properties together with sufficient corrosion resistance to intercrystalline corrosion.
The soft annealing is preferably performed in the continuous furnace at a metal temperature of 350° C.-550° C., preferably at 400° C.-450° C. for 10 seconds to 5 minutes, preferably 20 seconds to 1 minute. This allows the cold rolled strip to recrystallize sufficiently thoroughly and the corresponding properties with regard to the very good formability and the average grain size to be achieved reliably and economically.
Finally, the object indicated above is achieved by a component for a motor vehicle, composed of the aluminium alloy strip according to the invention. The components are characterised in that, as already stated, they can be deep-drawn with a large surface area and therefore for example large-area components for automotive construction can be provided. Furthermore, because of the strength properties provided these also have the necessary stiffness and the corrosion resistance required for use in automotive construction.
It is conceivable, for example, for the component according to a further configuration to be a motor vehicle body part or body accessory, which apart from being subject to high strength requirements is also heat-stressed. Preferably, the body-in-white parts such as an internal door part or an internal tailgate part, are made from the aluminium alloy strip according to the invention.
The invention is explained in more detail below with the help of the drawing. The drawing shows as follows:
a shows a top view of the specimen geometry for the plane-strain cupping test according to DIN EN ISO 12004.
b shows a side-view of the schematic test set-up for the plane-strain cupping test according to DIN EN ISO 12004.
The flow diagram of
In step 1, a rolling ingot of an AlMg aluminium alloy with the following alloying elements is cast, for example in DC continuous casting:
Then the rolling ingot in process step 2 undergoes homogenisation, which can be performed in one or more stages. During homogenisation, temperatures of the rolling ingot of 480 to 550° C. are reached for at least 0.5 hours. In process step 3 the rolling ingot is then hot rolled, wherein typically temperatures of 280° C. to 500° C. are reached. The final thicknesses of the hot-rolled strip are for example 2.8 to 8 mm. The hot-rolled strip thickness can be selected such that after hot rolling only a single cold rolling step 4 takes place, in which the hot-rolled strip, with a degree of rolling of 40% to 70%, preferably 50% to 60%, has its thickness reduced to the final thickness.
Then the aluminium alloy strip that has been cold-rolled to its final thickness undergoes soft annealing. According to the invention the soft annealing is performed in a continuous furnace. In the embodiments shown in Table 1, the second route was applied with an intermediate annealing. For this, the hot-rolled strip after hot rolling according to process step 3 is passed for cold rolling 4a, in which the aluminium alloy strip is cold rolled to an intermediate thickness, which is determined in such a way that the final degree of cold rolling to the final thickness is 40% to 70% or 50% to 60%. In a subsequent intermediate annealing the aluminium alloy strip preferably recrystallizes throughout. The intermediate annealing was carried out in the embodiments either in the continuous furnace at 400° C. to 450° C. or in the chamber furnace at 330° C. to 380° C.
The intermediate annealing is shown in
The aluminium alloy strips manufactured in this way had their mechanical characteristics determined, in particular the yield point Rp0.2, the tensile strength Rm, the uniform elongation Ag and the elongation at rupture A80mm, Tables 2, 5. Apart from the mechanical characteristics of the aluminium alloy strips measured according to EN 10002-1 or ISO 6892 in addition the average grain sizes according to ASTM E1382 in μm are given. Furthermore, the corrosion resistance to intercrystalline corrosion in accordance with ASTM G67 was measured, and in fact without additional heat treatment in the initial state (at 0 h). In order to simulate use in a motor vehicle, the aluminium alloy strips, prior to the corrosion test, furthermore underwent various heat treatments. A first heat treatment consisted of storage of the aluminium strips for 20 minutes at 185° C., in order to model the CDP cycle.
In a further series of measurements the aluminium alloy strips were also stored for 200 hours or 500 hours at 80° C. and then underwent the corrosion test. Since the forming of aluminium alloy strips or sheets can also affect the corrosion resistance, the aluminium alloy strips were stretched in a further trial by approximately 15%, and underwent heat treatment or storage at raised temperature and then a test for intercrystalline corrosion according to ASTM G67, during which the mass loss was measured.
Table 1 gives the alloying contents of a total of four different aluminium alloys, which fall within the specification of the AA5182-type aluminium alloy. The reference alloy is constituted by the material used to date and is shown in comparison to variants 1, 2 and 3. Table 1 also contains details of the type of final annealing, the final degree of rolling and the measured average grain size (grain size diameter) in μm. Variants 1 and 2 differ here merely in terms of final degree of rolling, which leads to the formation of a different grain size. Thus variant 2 differs from variant 1 irrespective of the almost identical alloying elements essentially in terms of the final degree of rolling of 57% at identical continuous furnace conditions. The result was that variant 2 had an average grain size of 18 μm compared to 33 μm for variant 1. The strips in Table 1 were heated in the continuous furnace for 20 seconds to 1 minute to a temperature of 400° C. to 450° C., then cooled and coiled at less than 100° C. The specimens taken were then, as indicated in Table 2, measured according to the corresponding DIN EN ISO standards.
It is clear from Table 2 that variant 1 in terms of the yield point does not reliably reach the value of 110 MPa and in the diagonal measurement, identified by the D symbol, has a value of less than 110 MPa. The measurement in the direction of rolling L and transversally to the direction of rolling Q showed, however, that variant 1 actually reached a yield point Rp0.2 of 110 MPa. The reference and variants 2 and 3 were significantly above this lower limit for the yield point. The embodiment according to the invention in variant 2 reliably achieved the yield point value of 110 M Pa in all tensile directions. It is clear to see that variant 3 with the highest Mg content of 4.95 wt. % achieves the highest yield point and tensile strength figures. It can also be seen that the different degree of rolling between variants 1 and 2 not only markedly influences the grain size, but in particular raises the yield point to a value of significantly higher than 110 MPa.
In particular the alloy according to the invention in variant 2 has a lower anisotropy compared to the reference, reflected in lower values of the planar anisotropy Ar. Here, the planar anisotropy Δr is defined as ½*(rL+rQ−2 rD), wherein rL, rQ and rD correspond to the r-values in the longitudinal, traversal and/or diagonal direction. Here, the average r-value F, calculated from ¼*(rL+rQ+2rD), does not differ significantly from that of the reference material.
Table 3 gives the measured values recorded in relation to the resistance to intercrystalline corrosion. It can be seen that variant 2 according to the invention in terms of the measured values of the reference, in particular in respect of the long-time stressing, has comparable values both in the stretched state and in the unstretched state. Here variant 2 and the reference are almost identical. Variant 3, which despite the having the highest yield point values and tensile strength values, nevertheless in the corrosion test demonstrated that an excessive Mg content results in an excessive mass loss, in particular in the long-time tests, which apart from a short temperature cycle of 20 minutes at 185° C. also include long-time stressing of 200 hours at 80° C.
With regard to the measured values in Table 3 regarding the formability it can be seen that in particular variant 2 was superior in terms of the stretch forming properties in the SZ32 cupping test and in the plane-strain cupping test to the reference alloy. The clear improvement in forming behaviour of the aluminium alloy strip according to variant 2 compared to the reference aluminium alloy strip shows that even with a reduced Mg content comparable yield point values and tensile strength vales could be achieved with the reference alloy, without major losses in resistance to intercrystalline corrosion. This was demonstrated in particular by the mass loss measurement performed according to ASTM G67 in the NAML test. Significantly, with variant 2 an improvement in the deep drawing behaviour in the Erichsen cupping test by 7% and in the plane-strain cupping tests by approximately 10% was found, demonstrating the additional forming potential of the aluminium alloy strip according to the invention. This additional forming potential can be used to produce deep-drawn, large-area sheet metal parts, for example internal door parts of a motor car.
A brief explanation of the test setup for the “SZ32 cupping” test according to DIN EN ISO 20482 and the plane-strain cupping test with Nakajima geometry according to DIN EN ISO 12004 is provided below.
a shows the geometry of test piece 1. From a circular sheet metal cut-out the tapered test piece 1 is cut such that the web 4 has a width of 100 mm and the radii 2 at the waisted parts are 20 mm. Dimension 3, which is 100 mm, represents the diameter of the punch.
The “SZ32 cupping” test according to Erichsen has a similar setup, but no wasted test pieces are used, however. Here, a test piece 9 is simply held between a holder 10 and a support 11 and drawn with a punch 12 until likewise a drop is measured in the load of the drawing force. Then, again, the corresponding position of the punch is measured. The opening of the dies in
In Tables 4 and 5, further embodiments and comparative examples were created and measured according to their mechanical characteristics and their resistance to intercrystalline corrosion. It can be seen that the combination of using the continuous furnace and a specifically selected grain size of 15 μm-30 μm, preferably of 15 μm-25 μm results in a good compromise between corrosion resistance and mechanical measured values. Thus, for example, the embodiments according to the invention Nos. 3, 4, 7 and 11 have a satisfactory resistance to intercrystalline corrosion and also exhibit the mechanical measured values Rp0.2 and Rm necessary for use in the automotive sector, so that these are ideal for the provision of large-area, deep-drawn components.
| Number | Date | Country | Kind |
|---|---|---|---|
| 12 181 356.2 | Aug 2012 | EP | regional |
| PCT/EP2013/064736 | Jul 2013 | EP | regional |
This patent application is a continuation of PCT/EP2013/067487, filed Aug. 22, 2013, which claims priority to European Application No. 12 181 356.2, filed Aug. 22, 2012, and PCT/EP2013/064736, filed Jul. 11, 2013, the entire teachings and disclosures of which are incorporated herein by reference thereto.
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/EP2013/067487 | Aug 2013 | US |
| Child | 14624793 | US |
