Method of manufacture of aluminium sheet of the AA5000 type

Abstract: In a method for the manufacture of aluminium alloy sheet having a content of Mg of 1.5 to 8.0 wt% and a total content of (Mn + Cr + Zr) of not more than 0.1 wt%, a solution heat treatment step is carried out at final thickness of the sheet in a continuous annealing furnace. In this solution heat treatment the heating rate of the sheet in the heating region of said continuous annealing furnace is at least 50°C/s. Induction heating may be used. Improved properties can be obtained.

Description: This invention relates to a method for the manufacture of aluminium alloy sheet having a content of Mg of 1.5 to 8 wt%, particularly having a composition belonging to the AA5000 series. The invention also relates to the sheet produced by the method, which is particularly suitable for forming applications for example in the manufacture of body parts by pressing in the automotive industry. In this specification sheet is not only taken to mean sheets obtained by cutting, but also strip-shaped sheet e.g. in coil form, which has not yet been cut. In aluminium alloys containing Mg as a main alloying element it is known to include at least one of Mn, Cr or Zr approximately 0.10-0.5 wt% (percentage by weight; herein all percentages are by weight unless otherwise specified). Mn, Cr and Zr are so-called dispersoid-forming alloying elements and their addition to the aluminium alloy obtains an effective grain refinement during annealing following cold-rolling. Other alloying elements may also be added for this purpose such as Co, Ti, Ni or V, but in practice Mn, Cr and Zr are the most used. A disadvantage is that these alloying elements have low solubility in aluminium. A further disadvantage is that these alloying elements are relatively expensive, while the value and the applicability of scrap material containing in particular Zr and/or Cr is low. Aluminium alloys containing Mg have also been disclosed in which the amounts of elements such as Mn, Cr and Zr are deliberately kept low. JP-A-5-345693 proposes an alloy having 6-10% Mg and a total of 0.01-0.2% of (Mn + Cr + Zr + V), which after rolling to final thickness is heat treated in a continuous annealing furnace at 450-550°C for less than 120s and then immediately cooled at 800°C/min or more to obtain a crystal grain size of 20-120µm. This heat treatment is as short as even 3s. EP-A-681034 also describes an alloy having low content of (Fe + Mn + Cr + Ti + Zr) which is subjected at final thickness to heat treatment by heating at a rate of at least 100°C/min to 450-550°C and maintaining the maximum temperature for not more than 300s. The cooling rate is 100°C/min or more. The temperature range is selected to achieve a desired recrystallization. The fastest heating rate mentioned is 540°C/min, but no particular reason is given for the selection of heating rate. EP-A-646655 also describes alloy sheets having low (Cr + Mn + Zr + V) which at final thickness are subjected to the heat treatment of heating at at least 3°C/s to 500-580°C, maintaining the maximum temperature for 0-60s and cooling at at least 2°C/s. It is said that if the temperature exceeds 580°C, the heating rate is less than 3°C/s or the temperature is maintained for more than 60s, abnormal grain growth occurs. The object of the present invention is to provide a method of manufacture of an aluminium alloy sheet having good formability in which the contents of Mn, Cr and Zr in particular are low. According to the invention there is provided a method for the manufacture of aluminium alloy sheet having a content of Mg of 1.5 to 8.0 wt% and a total content of (Mn + Cr + Zr) of not more than 0.1 wt%, comprising a solution heat treatment step at final thickness of the sheet in a continuous annealing furnace, characterised in that in said solution heat treatment the heating rate of the sheet in the heating region of said continuous annealing furnace is at least 50°C/s. It has unexpectedly been found that the use of a very high heating speed in the continuous annealing furnace, which can be achieved for example by homogenous heating by means of inductive heating, gives improved mechanical properties to the sheet. First the composition of the alloy of the sheet produced by the invention will be discussed. In the aluminium sheet the sum of the (Mn + Cr + Zr)-content is less than or equal to 0.1%. Typically therefore, deliberately no Mn, Cr or Zr is added to the alloy, other than what is by chance or inevitably present in the recycling scrap used. This achieves the effect that adding these relatively expensive alloying elements may be omitted, while at the same time the value and the applicability of the recycling scrap of the aluminium sheet produced in accordance with the invention increase. Preferably the average grain size after the solution heat treatment is in the range 20-80µm. In this context grain size is taken to mean the average grain diameter according to ASTM-E-112. This achieves the effect that the formability of the aluminium sheet is optimal. With a grain size smaller than approximately 20 µm, the formability of the aluminium sheet may sharply decrease, while it may also not be possible to suppress the formation of so-called Lüders lines. If the grain size in the aluminium sheet is larger than approximately 80µm, then a so-called orange peel can form during pressing. This is an undesired effect. The Fe-content is preferably less than 0.5% and more preferably less than approximately 0.25%. With an Fe-content larger than approximately 0.5%, relatively large intermetallic compounds may form which can greatly reduce the formability of the aluminium sheet. An Fe-content less than 0.25% achieves the effect that the aluminium sheet combines a good formability with an average grain size in the desired range. The Si-content is preferably less than approximately 0.4% more and preferably less than 0.2%. This achieves the effect that the aluminium sheet combines a good formability with an average grain size in the desired range. The Cu-content may be less than 1.0% and preferably in the range 0.3-0.7%. This achieves the effect that the aluminium sheet is thermally hardenable. Alternatively, Cu is in a range up to a maximum 0.20%. In this case the element Cu is to be regarded as an impurity and is not deliberately added to the alloy, other than what is present by chance or inevitably in the recycling scrap used. Ti may be present as grain refining element for the cast micro-structure. The Ti-content is preferably less than 0.2% and more preferably less than 0.1% in order to prevent the formation of coarse TiAl 3 particles during solidification. The Ti may be added in combination with the elements B or C and preferably the B content is in a range less than 0.01% to prevent the presence of TiB 2 particles in the structure. Zn may be present in a range up to a maximum 0.25%. Generally Zn is to be regarded as an impurity and is not deliberately added to the alloy other than what is present by chance or inevitably in the recycling scrap used. Zn can be tolerated in a greater range than other impurities but the Zn content is desirably limited to the stated range. Other dispersoid-forming elements, such as Co, Ni and V are also preferably only at impurity level, if present. In the method for the manufacture of the aluminium sheet in accordance with the invention the sheet may be brought to its final thickness (finished thickness at end of the rolling processes) by conventional rolling techniques. The aluminium sheet is then solution heat-treated so that recrystallization takes place, in a continuous annealing furnace. The heating rate of the aluminium sheet in the heating section of the continuous annealing furnace is at least 50°C/s, and is preferably over 80°C/s. The maximum temperature is preferably within the temperature range 450-570°C and the time at this temperature is preferably 5-300s, and more preferably 5-30s. Immediately following solution heat-treatment the aluminium sheet is preferably cooled to below 150°C at a cooling rate of at least 100°C/min. The method in accordance with the invention has many advantages. Firstly the mechanical properties of the aluminium sheet produced in accordance with the invention can be at least comparable to those of aluminium sheet of the AA5000 type alloy in the stated range of the Mg-content in which at least one of the alloying elements Zr, Cr or Mn is also present in a range greater than 0.1%. Secondly the formability of the aluminium sheet in accordance with the invention can be greater than an aluminium sheet of a similar type of alloy with about the same Mg-content, but with the presence of such alloying elements as Zr, Cr or Mn. This is achieved in part because the grain size is in the stated range attained with the heat-treatment in accordance with the invention. Furthermore, because of the very short annealing time virtually no Mg-oxide scale forms on the surface of the aluminium sheet, so that no discolouration and less problems with pressing occur. Also the rolling oil present following cold-rolling gives far less problems in continuous annealing because no staining occurs as a result of partial burning. Moreover the throughput time of the product is very short. Figs. 1 and 2 show graphically the test results of the following Examples. Example 1 Two types of aluminium sheet of different alloy compositions A and B belonging to the AA5000-series were cold-rolled to a finished thickness of 1.06mm and then solution heat-treated in two different processes I and II. Table 1 lists the chemical compositions A and B of the aluminium sheets, in which A is a comparative composition and B is an alloy composition in accordance with the invention. The alloy composition of the tested aluminium sheets (wt%) Sheet Mg Mn Cu Fe Si Zr Cr A 4.05 0.37 0.01 0.28 0.10 0.01 0.01 B 4.80 0.05 0.01 0.18 0.11 0.01 0.01 The two different processes of solution heat treatment are: I. heating the aluminium sheet at a heating rate of approximately 25°C/hour to a temperature of 360°C, then holding for 1 hour at 360°C, and then allowing to cool in air to below 150°C at a cooling rate of approximately 10°C/hour. This is a known method and is often designated by the term 'batch-anneal'. II. annealing the aluminum sheet in a continuous annealing furnace in which the aluminium sheet is heated by inductive heating at about 100°C/s, the aluminium sheet is held for 10 seconds at a temperature of 530°C, and then quenched to below a temperature of 150°C. Following these solution heat-treatments the uniform elongation Ag, the total elongation A80, and the work hardening exponent "n" of the four different aluminium sheet products were determined together with the grain size. The results are given in Fig. 1 and Table 2. In Fig. 1, A-I gives the sheet of composition A treated by treatment I, etc. It can be seen from these results that with the same alloy types the grain size of statically annealed material (treatment I) is smaller than of continuously annealed material (treatment II). At the same time it can be seen that with the same alloy types continuously annealed material has a higher Ag, A80 and "n" exponent than batch annealed material. When comparing Alloy A-I with Alloy B-I, and Alloy A-II with Alloy B-II it can be seen that best results of elongation and "n" exponent are obtained with the aluminium sheet of composition and the solution heat treatment in accordance with the invention, while the average grain size is in the desired range. The grain size of the different aluminium sheets following solution heat treatment Aluminium sheet ASTM grain size number average grain size (µm) Alloy A-I 10.3 10 Alloy A-II 9.2 15 Alloy B-I 8.0 25 Alloy B-II 7.2 30 Example 2 Three aluminium sheets with a chemical composition in accordance with plate B of Table 1, were cold rolled to a finished sheet thickness of 1.06mm and then solution heat treated in three different processes a, b and c. The three different processes were: a. heating the aluminium sheet at a heating rate of approximately 25°C/hour to a temperature of 360°C, then holding for 1 hour at 360°C, and then allowing to cool in air to below 150°C at a cooling rate of approximately 10°C/hour (batch annealing). b. annealing the aluminium sheet in a continuous annealing furnace in which the aluminium sheet is heated by inductive heating at a heating rate of approximately 5°C/sec, the aluminium sheet is held for 10 sec. at a temperature of 530°C, and then quenched to below at a temperature of 150°C at a cooling rate of approximately 150°C/sec. c. annealing the aluminium sheet in a continuous annealing furnace in which the aluminium sheet is heated by inductive heating at a heating rate of approximately 100°C/sec, the aluminium sheet is held for 10 sec. at a temperature of 530°C, and then quenched to below at a temperature of 150°C at a cooling rate of approximately 150°C/sec. Following solution heat treatment the Ag, A80 and the work hardening exponent "n" were determined as in Example 1. The results are shown in Fig. 2, in which B-a gives the results for sheet of composition B treated by process a, etc. From these results it can be seen that continuously annealed sheets (treatment b and c) have a higher Ag, A80 and "n"-exponent than batch annealed sheet (treatment a). It can also be seen that continuous annealed sheet with a heating rate of approximately 100°C/sec has a higher Ag, A80 and "n"-exponent than sheet with a heating rate of approximately 5°C/sec.

Claims: A method for the manufacture of aluminium alloy sheet having a content of Mg of 1.5 to 8.0 wt% and a total content of (Mn + Cr + Zr) of not more than 0.1 wt%, comprising a solution heat treatment step at final thickness of the sheet in a continuous annealing furnace, characterised in that in said solution heat treatment the heating rate of the sheet in the heating region of said continuous annealing furnace is at least 50°C/s. A method according to claim 1, wherein said heating rate is at least 80°C/s. A method according to claim 1 or 2, wherein in said solution heat treatment, the sheet is held at a maximum temperature in the range 450 to 570°C for a time period in the range 5 to 300s. A method according to claim 3 wherein said time period is in the range 5 to 30s. A method according to any one of the preceding claims, wherein in said solution heat treatment the cooling rate of said sheet is at least 100°C/min to below 150°C. A method according to any one of the preceding claims, wherein in said heating region of said continuous annealing furnace, the sheet is heated by induction heating. A method according to any one of the preceding claims, wherein said aluminium alloy belongs to the AA5000 series. A method according to any one of the preceding claims, wherein the average grain size of the aluminium alloy after the solution heat treatment is in the range 20 to 80µm. A method according to any one of the preceding claims, wherein said aluminium alloy has a content of Fe of not more than 0.5 wt%. A method according to any one of the preceding claims, wherein said aluminium alloy has a content of Si of not more than 0.4 wt%. A method according to any one of the preceding claims, wherein said aluminium alloy has a content of Cu or not more than 1.0 wt%. A method according to claim 11, wherein said aluminium alloy has a Cu content of not more than 0.2 wt%.

Inventors: BOTTEMA JAN, DE SMET PETER, KAASENBROOD PAUL HENDRIKUS THE, BOTTEMA, JAN, DE SMET, PETER, KAASENBROOD, PAUL HENDRIKUS THEODORUS
Applicants: CORUS ALUMINIUM NV, CORUS ALUMINIUM N.V.
Application year: 1997
Main classification: C22F1
IPC codes: C22C21, C22F1