Abstract:
During the process in which a metal strip is moved in a floating condition, the metal strip is first heated, and thereafter cooled and annealed. In cooling the metal strip, the metal strip is cooled so that both edges widthwise thereof are lowered in temperature more quickly than the temperature of a central portion widthwise thereof. The metal strip is cooled in a manner as described whereby a great thermal stress is not introduced in the metal strip and consequently, the metal strip is cooled without being wrinkled.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a method for cooling materials of low elastic limit such as aluminum strips, copper strips, and iron strips heated for the annealing. 
     2. Description of the Prior Arts 
     In prior arts, in the case where a metal strip (The term &#34;metal strip&#34; as used herein indicates a thin and lengthy band-like metal plate continuously rolled by a rolling mill. The thickness of the metal strip is normally less than 3.5 mm, and the plate has various widths.) is subject to cooling after it has been heated for heat treatment, even if the metal strip whose temperature has already decreased to a level below a definite value is cooled at a large temperature gradient, large thermal stresses are not introduced to the strip. However, when the temperature of the strip is not yet decreased to a level as noted above, cooling of the strip at a large temperature gradient produces large thermal stresses in the metal strip as shown by the broken lines in FIG. 5. When such large stresses occur, the stresses overcome the antibuckling stress of the metal strip and consequently give rise to wrinkles, in the metal strip, parallel to the moving direction thereof as shown in FIG. 7, resulting in a defective metal strip. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a cooling method which in cooling a heated metal strip, can minimize a stress produced in the metal strip. 
     That is, in the method of the present invention, when a metal strip is cooled, cooling is effected by controlling the temperature in such a manner that both edges of the metal strip are lower in temperature than a central portion thereof, and as a result, less thermal stresses are introduced in the metal strip. Thus, the metal strip can be finished into a product of good quality without wrinkles. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic longitudinal sectional view of a heat treatment apparatus; 
     FIG. 2 is a sectional view on a larger scale taken along the line II--II of FIG. 1; 
     FIG. 2--2 is a sectional view showing an embodiment in which the plenum chamber is differently divided; 
     FIG. 3 is a schematic perspective view showing a state wherein the metal strip is paid off and rewound; 
     FIG. 4 is a graphic representation showing changes in temperature of an aluminum strip; 
     FIGS. 5 and 6 are respectively graphic representations showing thermal stresses produced in the aluminum strip; and 
     FIG. 7 is a perspective view showing a state wherein wrinkles are produced in the metal strip. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to FIG. 1, heat treatment apparatus 1 consists of a heating apparatus 2 and a cooling apparatus 12. First, the heating apparatus 2 will be explained. As is known, a furnace wall 3 is designed so as to form a heat shielding between the interior and exterior thereof. The furnace wall 3 has a part formed with an entrance port 4 and a reception port 5. A metal strip 6 is inserted through the entrance port 4 and reception port 5 as shown. The furnace wall 3 has plenum chambers 7 and 7 provided in a space interiorly thereof. These plenum chambers 7 and 7 are disposed opposedly in a position through which the metal strip 6 passes, and the plenum chambers 7 and 7 have their surfaces opposed to each other provided with a plurality of gas blowing nozzles in a known manner. A recirculation fan 8 is mounted on the furnace wall 3. One end of a conduit 9 is communicated with the recirculation fan 8 and the other end thereof is communicated with the plenum chamber 7. The furnace wall 3 is further internally provided with burners 10. Entrance rolls 11 are disposed frontwardly of the entrance port 4. The entrance rolls 11 are provided to steadily guide the metal strip 6 towards the entrance port 4. It is noted that the plenum chambers 7, conduit 9 and the like in the heating apparatus 2 are designed substantially equally to those members as used in the cooling apparatus, which will be described hereinafter. 
     Next, the cooling apparatus 12 will be explained. The details of the cooling apparatus 12 are shown in FIG. 2. Plenum chambers 15 and 15 are disposed within a frame 13 formed of a metal material. These plenum chambers 15 and 15 are disposed opposedly in a position through which the metal strip 6 passes. These plenum chambers have their surfaces opposed to each other provided with a plurality of gas blowing nozzles in a known manner. The plenum chamber 15 is partitioned plurally widthwise of the metal strip 6 by means of partitioning walls 16, 16 . . . and divided into a plurality of chamber elements 15a, 15a, . . . as shown. 
     An air blower 18 is mounted on the frame 13. One end of the conduit 19 is communicated with the air blower 18 and the other end thereof is communicated with the plenum chamber 15. The conduit 19 is interiorly partitioned by partitioning walls 20, 20 . . . and divided into a plurality of conduit elements 19a, 19a . . . The partitioning walls 20, 20 . . . are individually continuous to the partitioning walls 16, 16 . . . of the plenum chamber 15 whereas the conduit elements 19a, 19a . . . are individually communicated with the chamber elements 15, 15a . . . The conduit elements 19a, 19a . . . are respectively provided with dampers 21, 21 . . . which control the quantity of air. 
     While the number of divisions widthwise of the plenum chamber 15 is five in the illustrated embodiment, it should be understood that a suitable number may be employed as occasion calls. The same is true for the number of divisions of the conduit. 
     Next, the operation of the apparatus will be described. A metal strip 6a wound as shown in FIG. 3 about a pay-off reel in a known manner is paid off as shown by the arrow 30. After the thus paid off metal strip 6 has been passed through various known devices, it is inserted through the above-mentioned heat treatment apparatus. The metal strip 6 issued from the heat treatment apparatus 1 passes through various known devices and thereafter, it is rewound about a rewind reel as shown at 6b in a known manner. 
     In a state wherein the metal strip 6 is inserted through the heat treatment apparatus as described above, the burner 10, and air blowers 8, 18 are operated. In the steady condition, the metal strip 6 between the plenum chambers 7, 7 and between chambers 15, 15 is held in a floating condition by hot gases (normal air not heated in the chamber 15) blown through the nozzles on said chambers. It will be noted that the air blowers, chambers and the like in the heating apparatus 2 and cooling apparatus 12 are designed so as to perform the aforementioned function and to increase and decrease temperatures of the metal strip 6 as will be described later. 
     The metal strip 6 passing through the heat treatment apparatus 1 while maintaining a floating condition as described above is heated by the heating apparatus 2 and then cooled by the cooling apparatus 12. In FIG. 1, a heating zone and a cooling zone are respectively designated at 25 and 26. 
     In the heat treatment of an aluminum strip as one example of the metal strip 6, one example of changes in temperature of the aluminum strip is shown in FIG. 4. 
     In the heating zone 25, the whole width of the aluminum strip 6 is heated at uniform temperature gradient. In the cooling zone 26, on the other hand, the aluminum strip 6 is cooled in a manner such that both edges 6&#39; are lowered in temperature most quickly as shown at (A), a central portion 6&#39;&#34; is lowered in temperature most gently as shown at (C), and intermediate portions 6&#34; therebetween are lowered in temperature at an about intermediate rate between the aforesaid (A) and (C) as shown at (B). This can be accomplished by adjusting openings of the plurality of dampers 21. That is, the adjustment of respective openings of the dampers 21 causes to decrease the quantity of air blown from the chamber element 15a opposed to the central portion widthwise of the strip, to increase the quantity of air blown from the chamber elements 15a and 15a opposed to both edges widthwise of the strip, and to make the quantity of air blown from the chamber element 15a opposed to the intermediate portion between the central portion and both edges of the strip nearly half. With this, the strip being cooled is lowered in temperature at both edges quickly as compared to the lowering of temperature of the central portion. The dimensions of various members are that the aluminum strip is 0.3 t×2000 W, the length of the heating zone 25 is 2.2  m, and the length of the cooling zone is 2.2 m. 
     When the aluminum strip is heated and cooled as previously described, thermal stresses as shown by the solid lines in FIGS. 5 and 6 are introduced in the edge and central portion, respectively, of the aluminum strip. In FIGS. 5 and 6, thermal stresses introduced in the aluminum strip when the latter is uniformly cooled over the whole width thereof are shown by the broken lines. As can be seen from FIGS. 5 and 6, in the case the aluminum strip is cooled in the cooling zone 26 in a manner such that both edges 6&#39; thereof are lowered in temperature quickly as compared to the central portion 6&#39;&#34;, a small thermal stress is introduced in the aluminum strip. This small thermal stress does not generally exceed the elastic limit of the aluminum strip. Accordingly, there is not produced so large a wrinkle as to impair a value of product in the aluminum strip. 
     The plenum chamber 15 has been divided into five sections widthwise of the strip in the illustrated embodiment. However, it should be noted that as shown in FIG. 2--2, the plenum chamber 15 may also be divided into three chambers, i.e., chamber elements 15b, 15b and 15b, opposed to the central portion and both edges, respectively, of the strip. In this case, the quantity of gases (air) blown from the chamber elements 15b is such that the openings of respective dampers 21b may be adjusted to increase the quantity of air blown from the chamber elements 15b opposed to the both edges of the strip and to decrease the quantity of air blown from the chamber element 15b opposed to the central portion of the strip. 
     It is understood that the term &#34;edges of the metal strip&#34; as herein used is intended to apply to the portions other than the central portion of the metal strip. That is, the term &#34;edges of the metal strip&#34; indicates the portions to which the outermost chamber elements of five chamber elements shown in FIG. 2 are opposed, portions to which both outer chamber elements of three chamber elements shown in FIG. 2--2 are opposed, and the like.