Method of making AlMn alloy strips or sheets and strip or sheet made of AlMn alloy

Description of the invention

The subject of the invention is a method of producing AlMn strips or sheets for the production of elements by soldering, and an AlMn strip or sheet produced by this method.

For example, heat exchangers for motor vehicles are usually made of aluminum sheets, with pre-fabricated heat exchanger elements such as plates, pipes and distributors joined together by brazing. The stresses which, in practical operation, act on the components thus produced installed in cars are significant due to shocks, prolonged vibrations, the effects of corrosion and similar factors. This is especially true for plates through which heat is dissipated.

Defects in these heat exchanger components due to the inadequate properties of the aluminum material can lead to significant damage. In the past, particular problems of this type have been caused by those areas of components where changes in the microstructure have occurred as a result of the action of heat during brazing.

For the reasons described above, in addition to good brazability, a high strength, in particular a high yield point Rp0.2, and toughness are required, even after brazing aluminum sheets of the type in question. Suitable aluminum sheets must simultaneously show good formability and excellent corrosion resistance.

The material for the production of heat exchanger plates, known from WO 97/18946, contains (percent by weight) 0.2 - 0.5% Fe, 0.7 - 1.2% Si, 1.2 - 1.6% Mn , <0.3% Mg, <0.05% Cu, <0.2% Zn, <0.1% Ti, unavoidable doping elements, the individual amounts of which are at most 0.05%, and their total amount is equal to at most 0.15%, with the balance aluminum. From this material, ingots are cast as a starting material and then preheated to a rolling starting temperature of at least 520 ° C and hot rolled. The subsequent cold rolling to the final thickness is carried out in at least two stages, with intermediate annealing to be carried out at an annealing temperature in the range of 360 - 400 ° C, between the cold rolling stages.

In practical tests of the material produced by the known method, it has been shown that the material properties of aluminum sheets produced according to the prior art are insufficient for certain applications. This applies in particular to strength and corrosion resistance, which still pose problems after brazing in the brazing joint areas. In addition, it has been shown, e.g. in the production of heat exchangers, that the possibilities of combining components obtained from the material known from WO 97/18946 with heat exchanger components made of another light metal type material are limited due to the corrosion potential difference which is too low.

Based on the known prior art described above, the object of the invention was to develop a method that allows the low-cost production of aluminum sheets which, even after brazing, reliably exhibit high strength, in particular a high yield point, and also excellent corrosion resistance.

This object is achieved with a method for producing AlMn strips or sheets for the production of components by brazing, which consists in the fact that

- starting material is prepared from a melt containing (in weight percent) 0.3 - 1.2% Si, <0.5% Fe, <0.1% Cu, 1.0 - 1.8% Mn, <0 , 3% Mg, 0.05 - 0.4% Cr + Zr, <0.1% Zn, <0.1% Ti, <0.15% Sn, unavoidable doping elements whose individual amounts are at most 0, 05% and their total amount is at most 0.15%, and the rest aluminum,

- this starting material is preheated to a preheating temperature to below 520 ° C, a residence time of not more than 12 hours,

- the preheated stock is hot rolled to form a hot strip or plate,

- the hot strip or plate is cold-rolled to form a cold strip or plate, without intermediate annealing, and

- the cold strip or plate is finally treated by annealing.

The invention is based on the composition of the melt used to produce the starting material, the composition of which is adapted in such a way that, in particular, the risk of intercrystalline corrosion and the corrosive effect are minimized.

Due to the fact that the corrosion pits are evenly distributed over the surface. As a result, it ensures high corrosion resistance.

The alloy used according to the invention and its processing parameters are simultaneously optimized in such a way that, at a hot-rolled temperature in the middle of the temperature range, without the need for intermediate annealing during cold rolling, an aluminum sheet with good quality can easily be obtained from this alloy. deformability and high strength, in particular with high values ​​of the elastic limit Rp0.2 and good elongation at break, even after brazing.

It has been found that, for the sheets according to the invention, the yield point Rp0.2 is at least 60 MPa after brazing. In many cases, a yield point Rp0.2 of at least 65 MPa can be achieved. The corrosion potential is generally below -750 mV, and in many cases even below -800 mV (measured against the NEK electrode according to ASTM G69).

The strength of the sheets after brazing, in the case of AlMn sheets according to the invention, is also positively influenced by the content of silicon. However, it has also been found that silicon simultaneously influences the occurrence of intergranular corrosion in interaction with tin. Thus, in the alloy used according to the invention, the predetermined range of the silicon content is selected in relation to the tin content so as to optimize the composition so that intergranular corrosion can be avoided. This ensures good corrosion resistance of the AlMn sheet produced by the method according to the invention, with high strength at the same time.

The latter property is especially achieved when the ratio of tin content [% Sn] to silicon content [% Si] in the molten material is <0.03, but the interaction between silicon and tin can be optimized even better if the ratio [ % Sn] / [% Si] & lt; 0.1.

In the preparation of the alloy, the addition of tin in the ratio indicated is necessary at least when the Si content of the melt is at least 0.75% by weight. However, the addition of tin in the given ratio is advisable even with an Si content of 0.5 wt.%. and above.

If the upper end of the range established for the Si content is limited to 1.0 wt.% At most, aluminum sheets with optimized high strength can be produced reliably by the process according to the invention and, moreover, the risk of intergranular corrosion can be minimized.

Iron accelerates the formation of primary phases that bind the silicon. Therefore, according to the invention, the iron content is limited to at most 0.5% by weight. This limitation of the iron content ensures that the silicon is kept in solution under the production conditions according to the invention. This can be ensured particularly reliably if the iron content is limited to at most 0.3% by weight.

The copper content of the alloy used according to the invention is limited to at most 0.1% by weight, preferably up to 0.05% by weight. Copper increases strength and also leads to a positive corrosion potential. However, the positive corrosion potential limits the possibilities for joining with other materials. Moreover, as the Cu content increases, the corrosion resistance deteriorates, especially with regard to intergranular corrosion.

The content of Mn in the melt according to the invention of at least 1.0 to at most 1.8% by weight increases the strength of the sheet according to the invention. Optimized strength properties can be reliably achieved when the Mn content in the melt is at least 1.3% by weight and at most 1.5% by weight.

Magnesium is added to the alloy used in the invention as a strength enhancing element. However, since the high magnesium content adversely affects the inert gas solderability (CAB brazing), according to the invention the magnesium content is limited to a maximum of 0.3 wt%. If particularly responsible brazing processes are to be carried out, limit the magnesium content to a maximum of 0.1% by weight. has a positive effect on the results of work.

The strength and corrosion resistance are further improved by adding Cr and / or Zr to the alloy used in the invention. If the total content of Cr and Zr is kept in the range of 0.05-0.4% by weight, this leads to the formation of a very durable microstructure (elongated, coarse grains) in which the intercrystalline corrosion process is slowed down due to the reduction of the grain boundary area. However, in combination with Mn, Fe and Ti, the presence of Cr and Zr can lead to the formation of thick precipitations, which in turn adversely affects the formability and strength of the sheets.

By the method of the invention. For this reason, in the alloy used according to the invention, the chromium and / or zirconium content increases with a low Mn content, while it decreases with a high Mn content.

The beneficial effect of Cr and / or Zr can be used particularly reliably when the content of Cr in the melt is from at least 0.1% by weight. % to at most 0.2 wt.% and the Zr content is at most 0.05 wt.%.

In order to avoid the negative influence of zinc on the corrosion of aluminum sheets of the type in question, the content of Zn is limited to 0.1% by weight, preferably to 0.05% by weight.

In order to refine the grain of the casting microstructure, titanium may be added to the alloy used according to the invention in an amount of up to 0.1% by weight, preferably up to 0.05% by weight.

In accordance with current practice, the starting material is continuously cast alloy processed ingots. However, as a starting material for the production of the AlMn sheets according to the invention, it is of course also possible to use a starting material produced in another way.

The method according to the invention enables the hot rolling to be carried out at a relatively low preheating temperature of the metal, below 520 ° C, which leads to an optimized microstructure of the hot strip with regard to formability and corrosion resistance. In order to ensure that the starting material is well cylindrical, the preheating temperature in this case is at least 400 ° C.

It is particularly advantageous if the starting material is preheated to a temperature of at most 470 ° C and the residence time during the preheating is limited to at most 5 hours in order to maintain the highest possible proportion of Mn in the solution. The manganese kept in the solution is finely dispersed during the subsequent annealing (softening / re-annealing) and during brazing, which in turn leads to the achievement of the desired high strength, in particular to the high yield strength Rp0.2. The starting temperature of the starting material during hot rolling is preferably at least 400 ° C for the reasons described above. In this case, the final rolling temperature during hot rolling is above 250 ° C, preferably above 300 ° C, which ensures sufficient formability of the starting material and, moreover, the formation of an optimized microstructure during hot rolling. The thickness of the hot strip is 2-10 mm.

The annealing carried out at the end of the process according to the invention is used to adapt to the delivery requirements. Annealing may then include soft annealing or re-annealing of the cold strip in a coil or in a continuous annealing furnace. When soft annealing is carried out, the temperature of the AlMn sheet during soft annealing is at least 300 ° C, preferably at least 350 ° C. The annealed strip is delivered to the manufacturer in the "0" state (softening annealed).

However, if the material is to be delivered tempered, e.g. H22 (work hardened, re-annealed, hardness 1/4), H24 (work hardened, re-annealed, hardness 2/4) or H26 (hardened by work, re-annealed, hardness 3/4), annealing is carried out as a re-annealing in a coil or in a continuous annealing furnace with appropriate temperature setting.

The typical thickness of the finished cold-rolled strip is 50 - 500 µm.

In order to further process the strip produced according to the invention, it may also be advantageous to clad the strip on one or both sides with one or two Al alloys, the thickness of the cover layer on each side being 3-20% of the total thickness of the strip.

As alloys, you can use e.g. typical solders, such as EN AW-4045, EN AW-4343, EN AW-4004, EN AW-4104 and their modifications, as well as typical protective layers, such as EN AW-1050, EN AW -1050A, EN AW-7072 and their modifications. The cover layer is in this case preferably applied by roll cladding.

The following examples illustrate the invention.

Table 1 shows the elemental composition of the alloy for AlMn 1-8 sheets.

PL 204 744 B1

Table 1

<td></td><td>Si</td><td>Fe</td><td>Cu</td><td>Me</td><td>Mg</td><td>Cr</td><td>Zn</td><td>Ti</td><td>Zr</td><td>Sn</td>

<td> 1</td><td> 0,89</td><td> 0,31</td><td> 0,011</td><td> 1,08</td><td> 0,001</td><td> 0,170</td><td> 0,006</td><td> 0,008</td><td> -</td><td> -</td>

<td> 2</td><td> 0,90</td><td> 0,30</td><td> 0,010</td><td> 1,05</td><td> 0,001</td><td> 0,005</td><td> 1,090</td><td> 0,007</td><td> -</td><td> -</td>

<td> 3</td><td> 0,55</td><td> 0,27</td><td> 0,009</td><td> 1,42</td><td> 0,031</td><td> 0,011</td><td> 0,007</td><td> 0,005</td><td> -</td><td> -</td>

<td> 4</td><td> 0,57</td><td> 0,30</td><td> 0,140</td><td> 1,07</td><td> 0,028</td><td> 0,116</td><td> 0,004</td><td> 0,006</td><td> -</td><td> -</td>

<td> 5</td><td> 0,84</td><td> 0,29</td><td> 0,008</td><td> 1,33</td><td> 0,063</td><td> 0,111</td><td> 0,005</td><td> 0,009</td><td> -</td><td> -</td>

<td> 6</td><td> 0,81</td><td> 0,31</td><td> 0,009</td><td> 1,37</td><td> 0,070</td><td> 0,123</td><td> 0,004</td><td> 0,005</td><td> -</td><td> 0,034</td>

<td> 7</td><td> 0,43</td><td> 0,31</td><td> 0,013</td><td> 1,03</td><td> 0,001</td><td> 0,015</td><td> 0,007</td><td> 0,008</td><td> -</td><td> -</td>

<td> 8</td><td> 0,74</td><td> 0,27</td><td> 0,014</td><td> 1,36</td><td> 0,083</td><td> 0,130</td><td> 0,004</td><td> 0,011</td><td> -</td><td> 0,089</td>

The content of the elements is given in percent by weight.

The ingots were continuously cast from an alloy of each of the following compositions. Subsequently, this ingot starting material was preheated to a preheating temperature which was 400-520 ° C, preferably 400-470 ° C.

The thus heated starting material was hot rolled with a final hot rolling temperature of at least 250 ° C, preferably 300 ° C, to obtain a hot strip 3.5 mm thick. The hot strip was then cold rolled in one or more passes to a final thickness of 100 µm. No intermediate annealing was performed during the cold rolling.

Finally, annealing was performed to accommodate delivery requirements, with either softening or re-annealing according to the manufacturer's instructions.

The cold-rolled strips were finally packaged as sheets.

The AlMn sheets produced in this way, softened in the delivery condition, had a yield strength Rp0.2 of not more than 80 MPa, a tensile strength Rm of at least 100 MPa and an elongation at break A100 of at least 3%.

Plates for the production of heat exchangers for car engines were made of AlMn 1-8 sheets (example numbers). The sheets could be cold formed with a bend radius of 1 mm for a bend angle of 180 °.

After fabricating the heat exchangers by brazing, each plate exhibited a yield strength Rp0.2 of at least 60 MPa, in many examples above 65 MPa, and a variable corrosion resistance. Tensile tests to determine the mechanical properties were in this case carried out on strip pieces subjected to a simulated soldering cycle. The brazing cycle was started at room temperature with a heating rate of about 25 K / min, a residence time at 600 ° C of 3 minutes, followed by cooling to room temperature at a cooling rate of about 40 K / min. Table 2 shows the values ​​of the elasticity limit Rp0.2 and the assessment of the corrosion resistance of sheets 1 - 8 in the state after brazing.

Table 2

<td rowspan="2"></td><td colspan="3"></td><td colspan="4">Condition after soldering</td>

<td></td><td></td><td></td><td>Rp0.2 [MPa]</td><td>_Rating¹⁾</td><td>Corrosion resistance ²⁾</td><td>Corrosion resistance intercrystalline²⁾</td>

<td> 1</td><td></td><td></td><td></td><td> 65</td><td> 7</td><td> 4,0</td><td> 2,5</td>

<td> 2</td><td></td><td></td><td></td><td> 62</td><td> 2</td><td> 2,5</td><td> 1,5</td>

<td> 3</td><td></td><td></td><td></td><td> 64</td><td> 13</td><td> 4,5</td><td> 4,0</td>

<td> 4</td><td></td><td></td><td></td><td> 66</td><td> 9</td><td> 3,0</td><td> 3,0</td>

<td> 5</td><td></td><td></td><td></td><td> 69</td><td> 8</td><td> 4,0</td><td> 3,0</td>

<td> 6</td><td></td><td></td><td></td><td> 70</td><td> 11</td><td> 4,0</td><td> 4,0</td>

<td> 7</td><td></td><td></td><td></td><td> 60</td><td> 14</td><td> 5,0</td><td> 4,5</td>

<td> 8</td><td></td><td></td><td></td><td> 70</td><td> 15</td><td> 4,5</td><td> 5,0</td>

¹⁾ 15 = excellent; 1 = very poor²⁾ 5.0 = excellent; 1.0 = very poor

PL 204 744 B1

It should be emphasized that the sheet 5, which did not contain tin at the Si [% Si] content of 0.84% ​​by weight, showed much worse corrosion resistance than the sheet 6 with a similar composition, in which the Sn [% Sn] content was 0.034% by weight. with an Si [% Si] content of 0.81% by weight, so that the ratio [% Sn] / [% Si] for sheet 6 was 0.042. In the case of sheet 8, which had an even better corrosion resistance in the brazed state, the ratio [% Sn] / [% Si] was 0.120. As shown by the results for sheet 7, with the content of Si [% Si] 0.43 wt.%. and tin-free, very good corrosion resistance can also be achieved with a low Si content. However, this does not lead to the high value of the yield point Rp0.2, as is achieved, for example, with sheets 6 and 8 with a higher Si content. Moreover, attention should be paid to the unfavorable influence of Cu (sheet 4), especially Zn (sheet 1), on corrosion resistance.