Patent Publication Number: US-2011048591-A1

Title: Method for heat treating a rolling stock made of a heat-treatable aluminum alloy

Description:
1. FIELD OF THE INVENTION 
     The invention relates to a method for heat treating a rolling stock, particularly a strip, made of an age-hardenable aluminium alloy, particularly an alloy of the 6000 series, wherein the rolling stock in the process of passing through an apparatus is fed to a solution annealing operation and a quenching operation, and in a subsequent step the rolling stock is subjected to hot exposure for age hardening. 
     2. DESCRIPTION OF THE PRIOR ART 
     In order to change the material properties of a strip made of an age-hardenable aluminium alloy, or to enable age-hardening of the same, a method is known from the state of the art (DE 24 46 828 B2) in which the strip is solutionized, then quenched and subsequently subjected to hot exposure. Various temperature and time information is known for such hot exposure, which also includes a two-step hot exposure of the strip rolled up into a coil, namely with the help of a bell-type annealing furnace and with the help of a heat bath in the next following step. The disadvantage in the known methods for hot exposure is that in addition to the comparatively high energy costs which are caused by the comparatively long duration of hot exposure, there are comparatively considerably deviating properties of the material also over the length of the strip. Such process variances in particular do not represent any quality feature of the strip, leading to disadvantages in competition. 
     It is further known from DE 24 46 828 that with different treatment times during solution annealing, quenching and hot exposure as well as with respectively different temperature progressions it is possible to influence the material properties of age-hardenable aluminium alloys. The levelling out of material properties over the entire length of such strips is not known from the state of the art. 
     It is further known from the state of the art (DE69628044T2) to heat rolling stock which is in passage to 65 to 121 degrees Celsius after quenching and to cool the same again to room temperature subsequently. Stabilization annealing known both from JP2007239005A and EP 1195449A2 is thus to be performed in order to achieve advantages in paint baking or to improve a so-called “paint-bake response” of aluminium sheet. Stabilization annealing does not offer any advantages in hot exposure for hot age-hardening. 
     SUMMARY OF THE INVENTION 
     The invention is therefore based on the object of improving a method for heat treating a rolling stock made of an age-hardenable aluminium alloy or a respective installation on the basis of the initially mentioned state of the art in such a way that it is not only possible to achieve a reduced treatment duration of the rolling stock with constant material properties, but that also the process variance in the heat treatment of the rolling stock remains at a low level. 
     This object is achieved by the invention with respect to the method in such a way that in a first step during the hot exposure the rolling stock which is in the process of passing through the apparatus is heated, whereupon in a further step the heated rolling stock is further subjected to the hot exposure. 
     If the rolling stock which is in the process of passing through the apparatus is heated during hot exposure as a first step and if thereupon the heated rolling stock is further subjected to hot exposure, it was surprisingly noticed that the material properties of the age-hardenable aluminium alloy can be leveled out with respect to its variance at least in sections over the length of the rolling stock. Examinations of the material of age-hardenable aluminium alloys of the 6000 series have shown that the known process variance in the heat treatment of these alloys can be reduced, by means which the invention especially stands out as compared with other known methods. Especially these alloys of the 6000 series in particular seem to respond to the method in accordance with the invention such as the 6061 (AlMg1SiCu) and 6082 (AlSi1MgMn) alloys. It can thus be seen that as a result the consolidation values are not only leveled out over the length of the rolling stock but can also be improved. An additional factor is that the duration of the further hot exposure of the rolling stock can be reduced, so that a reduced energy input can be expected in addition to an increase in the production capacity. The rolling stock can thus already be prepared during the passage for subsequent age hardening or it can already be begun at this point, with the rolling stock heated in the passage being subjected in the hot state to a further hot exposure or age hardening and is thus completed. The known negative effects of intermediate storage from quenching to age hardening can advantageously be avoided. Moreover, the heat or supplied energy from the first step of hot exposure can at least partly be used for further hot exposure. The method in accordance with the invention for age hardening can thus be more efficient and also cost-effective in its entirety in comparison with known methods. A large variety of known temperatures from the state of the art can be used for age hardening, which temperatures can be between 120 and 250 degrees Celsius. 
     Advantageous conditions for the method can be obtained concerning the heating of the passing strip when the rolling stock passes through a heating which occurs over the width of the rolling stock in sections over the length of the rolling stock as a first step of hot exposure, and it is then further subjected to hot exposure either cut to size or coiled into a coil. Moreover, this can enable a rapid heating of a rolling stock, which can lead to advantageous material properties. 
     If the rolling stock which is in passage in the apparatus is heated to one of its age-hardening temperatures, preferably 160 degrees Celsius, this can lead to considerable improvements in the material properties of the rolling stock after subsequent age hardening. It is then possible to bring the rolling stock to a state of age-hardening already in the first step, which can improve further age hardening. 
     If the rolling stock heated in the first step is further subjected to hot exposure prior to its cooling to a temperature beneath the temperature for hot exposure, especially beneath its lowest age-hardening temperature, it can be ensured that cold aging of the rolling stock cannot occur, which can lead to an advantageous hardness progression during hot exposure. Known temperatures of aluminium alloys are from 120 degrees Celsius for this purpose, so that approx. 120 degrees Celsius can be regarded as the lowest age-hardening temperature during hot exposure. 
     When the rolling stock is introduced into a heatable furnace for further hot exposure, process conditions for improved material properties can be provided. It is also possible to raise the temperature of the rolling stock as a result of heating by the first step of hot exposure with the help of the heatable furnace and to thus provide a two-step heating to exposure temperature. As an alternative to this, the rolling stock can be encompassed at least partly by a device for reducing its heat radiation, so that it is possible to omit a further additional heat source in respect of the furnace because the rolling stock can be further heat-exposed or age-hardened as a result of its sensible heat. This method can be especially energy-efficient. 
     It has proved to be especially positive when solution annealing of the rolling stock occurs at 450 to 570 degrees Celsius, the quenching of the rolling stock is made to less than 200 degrees Celsius, preferably to room temperature, and the heating in the first step of hot exposure is made to 100 to 200 degrees Celsius, which was shown for example in tests with aluminium alloys of the 6000 series. Preferably, heating in the first step to 160 degrees has proven to be positive for age hardening because this led to a special response in the alloy of the 6000 series. 
     Moreover, the strength of the aluminium alloy can be improved even further in that the time interval between the end of the quenching and the start of heating of the first step of the hot exposure is less than 60 minutes. Tests with aluminium alloys of the 6000 series led to a considerable increase in the strength by these parameters, wherein it was possible to reduce the time for hot exposure or age hardening to a considerable extent. 
     The object with respect to the installation is achieved by the invention in such a way that the apparatus is associated at least partly with the heating of the device for hot exposure of the rolling stock. 
     If the apparatus is associated at least partly with the heating of the device for hot exposure of the rolling stock, it is then possible to provide not only special process conditions but also that the installation can meet compact constructional conditions. 
     Simple constructional conditions are obtained when the heating associated with the apparatus belongs to a continuous furnace, with rolling stock which is in passage through the apparatus passing through said furnace as a first step of hot exposure. 
     Moreover, the constructional conditions can be improved when the other parts of the device for hot exposure of the rolling stock are arranged separate from the apparatus. 
     If the apparatus for heating the rolling stock comprises the entire heating of the device for the hot exposure of the rolling stock to hot exposure temperature, especially age-hardening temperature, with the other part of the device for hot exposure of the rolling stock comprising an insulating chamber which at least partly encompasses the rolling stock for age-hardening the rolling stock, an especially energy-efficient installation for age hardening of a rolling stock can be achieved among other things. The rolling stock is brought rapidly to the temperature for hot exposure, which occurs while still in passage, and is then subsequently further age-hardened in the heated state and without any additional further energy input. Moreover, advantageous material properties are further enabled with such an installation, as has already been described above. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The subject matter of the invention is shown by way of example by reference to embodiments, wherein: 
         FIG. 1  shows a schematic illustration of an apparatus for performing the method for heat treatment; 
         FIG. 2  shows a schematic illustration of a further embodiment of an apparatus for performing the heat treatment; 
         FIG. 3  shows an abridged and schematically indicated top view of the part of the device for hot exposure of the rolling stock which is associated with the apparatus according to  FIG. 1  or  2 ; 
         FIG. 4  shows an insulating chamber in a partly sectional side view for performing the further steps for hot exposure, and, as an alternative thereto, 
         FIG. 5  shows a heatable bell-type furnace in a partly sectional side view. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The method in accordance with the invention will be explained by reference to the apparatus  1  as shown in  FIG. 1  by way of an example. In accordance with apparatus  1 , a rolling stock  3 , especially a strip, made of an age-hardenable aluminium alloy is rolled up into a coil  2 , which rolling stock  3  is guided or pulled through a continuous strip furnace  5  via a deflection roller  4  for solution annealing as a first process step. Appropriate temperatures of solution annealing lie between 450 and 570 degrees Celsius. The rolling stock  3  then passes through a quenching apparatus  7  for quenching as a second method step, with which the rolling stock  3  is cooled off, preferably to room temperature. After the cooling of the rolling stock  3 , the rolling stock  3  is rolled up into a coil  8  again via a deflection roller  6  and is subjected to hot exposure as the last method step in the heat treatment for age hardening. The hot exposure temperatures and times must be adjusted to the respective alloy and the application in order to thus achieve the desired properties of the rolling stock  3 . Appropriate temperatures for such hot exposure are between 100 and 200 degrees Celsius for example. The strength of the aluminium alloy can be set for example by means of the parameters of the hot exposure or age hardening. In accordance with the invention however, the first method step of hot exposure is already performed by the apparatus  1 . A device for heating, which is a further continuous furnace  10 , is provided between the device  7  for quenching and the coiling device  9 , which furnace  10  heats the cooled rolling stock  3  in sections, which is shown especially in  FIG. 3 . With the help of the continuous furnace  10 , the rolling stock  3  thus already passes through a heating  11  over the width of the rolling stock in sections of the length of the rolling stock while disposed in apparatus  1 . For this purpose, the continuous furnace  10  comprises a heating  12  which can act on the rolling stock  3  inductively, via infrared or with the help of a gas flame. The relevant aspect is that the rolling stock  3  is heated again over the temperature by cooling, which occurs in passage of apparatus  1 . In this method step, the rolling stock  3  which is in passage in apparatus  1  and has already been cooled off, especially to room temperature, is heated again with the help of the apparatus  1 , whereupon the rolling stock  3  is further subjected to heat exposure either then or in a further step. The rolling stock  3  thus no longer cools off to its temperature before heating, but is further subjected in the heated state of hot exposure to age hardening. In contrast to the state of the art, the hot exposure has already started by apparatus  1 , which thus not only accelerates the method for heat treatment in its entirety, but also simultaneously ensures an even distribution of strength over the entire length of the rolling stock  3 . An additional factor is that thus higher strength values can be achieved than a rolling stock  3  would show which was produced according to the known method. Moreover, tests have shown that the process variance during the heat treatment of the rolling stock  3  can be kept to a low level. Heating to 160 degrees Celsius has proven to be especially advantageous, one of the temperatures for age-hardening which are classified according to the state of the art to range from 120 to 250 degrees Celsius. If cooling beneath one of its temperatures for age-hardening is prevented and the hot exposure has already begun in this state, further advantageous process conditions were observed. 
     It is possible that the rolling stock  3  is further subjected to hot exposure by not being wound up into a coil  8 , but cut to size, e.g. in plates. Coiling of the rolling stock  3  into a coil  8  appears to be advantageous because in this case the heat radiation of the sections thus heated and then coiled will act predominantly in the direction of the interior of the coil. 
     The further embodiment of an apparatus  1  as shown in  FIG. 2  differs from the apparatus  1  shown in  FIG. 1  in such a way that the deflection roller  4  and the deflection roller  6  were replaced by a clamping device  13  and an unclamping device  14 . These devices allow a continuous feed of the apparatus  1  or a continuous passage of the rolling stock  3  made of an age-hardenable aluminium alloy. For this purpose, the devices  13  and  14  are displaceably mounted, so that the apparatus  1  can be supplied with an endless strip. Similarly, shears  15  and  16  for cutting the endless strip into size are provided. 
     The device for the further hot exposure of the rolling stock  3  can comprise a bell-type furnace  17 , which is shown according to  FIG. 5 . As an alternative to this, the rolling stock  3  can also be partly encompassed by a device for reducing the heat radiation, especially an insulating chamber  18 , which is shown according to  FIG. 4 . As a result, the other parts of the device for hot exposure of the rolling stock  3  are arranged separate from the apparatus  1 . 
     The insulating chamber  18  comprises an openable door  19  which is movably articulated on the housing  20 , so that upon opening the same the coil  8  to be further exposed to heat can be introduced into the insulating chamber  18 . The insulating chamber  18  is lined in its inner region with a thermal insulation  21 . The advantage over known heatable furnaces such as bell-type furnaces  17  is that the further heat exposure occurs in an insulating chamber  18  exclusively by the sensible heat of the coil  8 , thus reducing operating costs. This requires however that the heating of the rolling stock occurs already at age-hardening temperature in the first step of hot exposure.