GB2167170A - Heat treatment of coils of metal - Google Patents

Abstract

Metal, e.g. Al, strip in coils is heat-treated by impinging jets of a temperature-controlled gaseous medium on the ends of the coils which are positioned with the coil axis horizontal, such that heat flow in the coil is by conduction along the axis. A rotating hearth furnace from such heat-treating has an annular hearth (12) which carries coils in intermittent movement through successive heat-treatment stations. At each station the gaseous medium is circulated by a fan or blower (31) to pass over heat-exchanger tubes (35) in chambers (26,27), then onto the coil ends via apertured plates 27 (28,29) forming the jets, and back to the chambers (26,27) through a cross-header (32) with inlet (33). The tubes (35) in a first series of stations are supplied with fluid heated by combustion or electrically, and those in a following series of stations are supplied with cooling medium. <IMAGE>

Description

SPECIFICATION Heat treatment of coils of metal Field of the invention: The invention relates to the heat-treatment of metal strip in the form of coils. It is particularly concerned with apparatus and a method for this purpose, more especially for continuously heat-treating a succession of such coils, of aluminium or of other metals.

Background of the invention: It is often desired to heat-treat metal strip in coil form under controlled atmosphere conditions. Previously this was accomplished in the case of steel strip by placing a coil end-on on a stool, or a stack of coils end-on, one on top of the other, closing them with an openbottom cover, usually called a bell, and heating the stool, coil or coils and bell in a furnace. The bell-was sealed with respect to the stool during the heating. The desired atmosphere was maintained by introducing an appropriate gas into the bell under a pressure above that prevailing in the furnace. The coils were spaced from each other and the stool by separators in grating form so as to allow circulation of the atmosphere in the bell. Aluminium, however, is much softer than steel.Aluminium coils cannot be heated at temperatures approaching full anneal (about 391 degrees C.) standing on their ends or an gratings without incurring severe damage to the wraps of the coil. Historically, therefore, aluminium in coils has been heat-treated by placing each coil on its side on a car, pulling a string of such loaded cars into a long furnace and raising or lowering the furnace temperature to obtain the desired heat-treating cycle.

Both processes above-mentioned require that the furnace and its entire contents be raised to the desired heat-treating temperature, held at that temperature for the required length of time and cooled to the desired discharge temperature for each furnace charge.

The cycle is then repeated for the next furnace charge. Such processes cannot be economical of heat.

Lee Wilson U.S. Patents 3,063,878 of November 13, 1962 and 3,114,539 of December 17, 1963 disclose an improved practice and apparatus for coil annealing. The coils to be treated were first uncoiled and then rewound to leave space between adjoining wraps through which a heated gaseous medium could readily pass and transfer heat to the coil wraps by convection. Coils so wound were called "open" coils. That winding and re-winding had to be carried out with the coil axis vertical so that the spacing between wraps would be reasonably uniform and the coil had to be supported on end during its heat-treating. An open grating type of coil stool was employed. The coils could be treated as before under bells on a base or in a rotary heath furnace as is disclosed in the '878 patent.In that way the time required for heat-treating and the amount of heat required was considerably reduced but the uncoiling and re-coiling operation, which had to be performed twice, once before and once after heat-treatment, and the necessity for handling the coils in upright position only were both undesirable and relatively costly.

Verwohlt U.S. Patent 2,613,070 of October 7, 1952 deals with heat-treating of metal strip in coil form in which a coil is loaded onto a horizontal mandrel and rotated thereon -in a furnace. The coil and the portion of the mandrel within it, are enclosed in a cover into which a protective atmosphere is introduced.

The weight of the coil causes those wraps below the mandrel axis to separate, and that separation is augmented by the differential expansion of the outer coil wraps with respect to the inner wraps, so that heating of the coil wraps by convection is facilitated. While Verwohlt's invention would minimize damage to coil ends, it would require the entire furnace to be heated and cooled, and the loading and unloading of coils into and out of the furnace would be time-consuming.

The procedures outlined above are not well adapted for coils of aluminium strip.

Summary of the invention The invention provides a method, and apparatus for performing the method, which includes projecting a temperature-modifying gaseous medium upon an end of a coil so as to cause heat flow in the coil by conduction in the direction of the coil axis. Unwinding and re-winding of coils may be omitted. The invention allows the coils to be treated with the coil axis horizontal. Heat transfer by conduction proceeds rapidly from the ends of a coil along a wrap toward the middle compared with heat transfer along a radius of a coil, from a wrap to the adjoining wrap.

Preferably heat is introduced into a coil from each end over the entire cross-section of the coil. This procedure is especially advantageous for coils of aluminium strip as the heat conductivity of aluminium is high compared to that of steel, but our invention may also be advantageous for coils of metal other than aluminium. Suitably the bulk velocity of projected medium is from 15 to 100 meters per second.

The invention also permits heat-treating of metal strip in coil form in a continuous manner, in which coils are, if necessary automatically, introduced in succession to serially disposed treating stations by indexing means which cause each coil to remain for a predetermined period of time at one station and then move it into the next succeeding station.

In the various stations, which can be held at substantially constant temperatures, the de sired heating, soaking and cooling stages of the cycle can be carried out. Most advantageously an annular rotable hearth furnace for heat-treating coils in the above manner can be used. Other preferred features of the apparatus are set out in the appendant claims and the following description.

DRAWINGS Figure 1 is a horizontal section through a furnace along line l-l of Figure 2 having a furnace chamber according to the invention; Figure 2 is a vertical section along line ll-ll of Figure 1; Figure 3 is a vertical section along line Ill-Ill of Figure 1; Figure 4 is an enlarged partial vertical-section through a portion of the furnace of Figure 2 along line IV-lV of Figure 2; and Figure 5 is an enlarged detail section of an aperture of Figure 4 along line V-V of Figure 4.

Description of preferred embodiment Figures 1 and 2 show a furnace comprising a stationary annular shell 11 with a rotating hearth 12. The shell has a cylindrical outside wall 13, a cylindrical inside wall 14 and a flat annular roof 15 extending between walls 13 and 14. The foundation 16 for the walls 13 and 14 also supports a circular track comprising an outer rail 17 and an inner rail 18 disposed between the walls 13 and 14. The hearth 12 travels on the track comprising the two rails 17 and 18. Flanged wheels 20 on stub shafts journalled in brackets 21 roll on the rails 17 and 18. The spaced brackets 21 are affixed in pairs to the lower flanges of cross beams 22 which are uniformly spaced from each other arcuately on radii of the shell 11 and carry the annular floor 23 of the hearth 12.

On the annular floor 23 there are positioned coil holders 24 uniformly spaced angularly around the floor 23 and shaped to support a coil which rests on its circumferential surface with the axis of the coil center on a radius of the shell 11. In the sections normal to the radius the coil holder 24 has a concave upward facing coil engaging profile. It is preferred to employ separate coil holders for each different diameter of coil to be treated.

For example a 60-inch diameter coil would be supported on a coil holder 24 having a concave upper surface with a radius of 30 inches or slightly more. The weight of the coil holds it in position on such a coil holder 24 without any fastening device.

Spaced around the inner face of the outside wall 13 are distribution chambers 26 and spaced around the face opposite those chambers 26 along the inside wall 14 there are similar distribution chambers 27. Each chamber 26 is positioned opposite the outside end of a coil holder 24 when the hearth 12 is stationary and each chamber 27 is positioned opposite the inside end of the coil holder. As will be explained hereinafter there is one less pair of distribution chambers 26 and 27 than the aggregate number of coil holders 24. The distribution chambers are essentially oblong boxes having flat opposing faces 28 and 29 respectively parallel to the ends of a coil positioned between the opposite chambers 26 and 27. Each opposing face 28 and 29 is formed with a pattern of holes 30 disposed over an area corresponding to the end face of a coil, as is shown in Figure 4.The holes 30 have rounded edges 45 on the inside of the chambers 26 and 27 as is shown in Figure 5.

Each distribution chamber 26 is connected at its upper end to a separate cross header 32 which is positioned against the lower surface of the chamber roof 15. The other end of the cross header 32 is connected to the opposite distribution chamber 27 at its upper end. At the center of each cross header 32 is an intake opening 33 opening into the furnace chamber. Within each cross header 32 at opening 33 is positioned a suction fan or blower 31 driven by a motor 34 mounted on the roof 15 outside the furnace chamber.

Within most of the distribution chambers 26 and 27 are located heat exchangers 35, the ends of which project through the roof 15 so as to be connected to heating sources or cooling sources, not shown.

A loading ånd unloading door 37 (Figure 1) is provided in the outer wall 13 of our furnace. A loading and unloading station 38 is defined at that doorway by doors 39 and 40 within the furnace chamber positioned crosswise thereof, one on each side of the door 37. If the heating cycle for which the furnace is employed includes a heating stage and a cooling stage a door 41 similar to the doors 39 and 40 is provided at another location in the furnace chamber as will be made clear hereinafter. The doors 37, 39, 40 and 41 are positive seal doors that are gas-tight when closed and which open and close by sliding vertically. Gas-tight seals 42 are also fitted between the rotary hearth 12 and the furnace shell 11. Those seals are of conventional type comprising a vertical rib 43 affixed to the lower edge of the shell 11 moveable in a trough 44 affixed to the hearth 12, the trough 44 being filled with sand or other granular material, or water or other fluids.

The hearth 12 is rotated on its circular rails 17 and 18 by a circular rack 48 affixed to the bottom of cross beams 22 which meshes with a pinion 49 (Figure 3). The pinion 49 is affixed to a horizontal shaft 50 journalled in bearing blocks 51 on the foundation 16 which shaft 50 is connected to a reducing gear box 52. The gear box in turn is driven by a motor 53.

The furnace comprises a loading and unloading station 38 (Figure 1) and fifteen heat treating stations spaced uniformly around the remainder of the 360 degree circumference of the furnace. Generally speaking, the stations between the doors 40 and 41 in the direction of hearth rotation shown in Figure 1 are heating stations and those between doors 41 and 39 are cooling stations. The rotary hearth 12 carries 16 coil holders which are uniformly spaced, each holder holding one coil. Each station is provided with an outer wall distribution chamber 26 and an inner wall distribution chamber 27, opposite each other and opposite the ends of any coil positioned between them as described previously. The number of heat-treating stations depends primarily on the size of the coils and the nature of the heattreating cycle.The atmosphere in all heating stations, between the doors 40 and 41, is the same. The atmosphere in all cooling stations, between the doors 40 and 39, is the same but not necessarily the same as the atmosphere in the heating stations.

The hearth 12 is caused to rotate intermittently. A coil loaded in the loading station 38 at ambient atmosphere is held there with the doors 37, 39 and 40 closed until the atmosphere in that station is modified to correspond to that in the heating stations the station acting equivalent to an airlock. Then the door 40 together with the doors 39 and 41 are raised and the hearth 12 is rotated to bring the coil previously loaded to the first heating station.

The door 40 together with the doors 39 and 41 are lowered and the hearth 12 is stopped and remains stationary for a pre-determined period of time. The atmosphere in the heating station is circulated by the suction fan 31 as has been described. The heat exchangers 35 heat the atmosphere and the heated gaseous atmosphere is projected against the ends of the coil through the apertures 30 in faces 28 and 29 of the distribution chambers 26 and 27 respectively. At the end of the pre-determined period of time, the hearth 12 is again caused to rotate sufficiently to bring the coil into the next adjoining heating station. The coil is brought up to the desired temperature in that way through moving to several heating stations depending upon the pre-determined time period, which is maintained constant.The heat input from the heat exchanger in each station is adjusted to bring the coil to the desired temperature at the desired time. The coil is then transferred to the soaking section of the furnace, in which the heat inputs are adjusted to render the coil temperature uniform at the desired level. When that uniformity has been achieved in the last soaking station, the door 41 together with the doors 39 and 40 are raised and the hearth carries the coil into the first of the cooling stations.

Here the heat exchangers are used to reduce the temperature of the atmosphere, which again is circulated and projected against the coil ends a,s jets which impinge on the coil ends.

The heat exchangers may be heated by combustion or electrically. In the cooling station, or some of them, ambient air of water circulation through the heat exchangers may provide sufficient cooling. In all stations the atmosphere, heated or cooled, is circulated as has been mentioned and is projected in the form of jets against the ends of the coil through the apertures 30 in the faces 28 and 29 of the distribution chambers 26 and 27 respectively. Jets having bulk velocities in the range of from 15.24 to 91.44 meters per second are satisfactory for annealing aluminium strip in coil form. The size and number of the apertures 30 depends on the pressure developed in the distribution chamber, the arrangement of the apertures, which should correspond to the coil cross-section as is shown in Figure 4, and the spacing between the faces 28 and 29 and the coil ends.

Claims (21)

1. A method of altering the temperature of a coil of metal strip including projecting a temperature-modifying gaseous medium upon an end of a coil so as to cause heat flow in the coil by conduction in the direction of the coil axis.

2. A method according to claim 1 in which the medium is projected in the form of jets.

3. A method according to any of the preceding claims in which the coil is horizontally positioned.

4. A method according to claim 3 in which the medium is projected simultaneously upon both ends of the coil.

5. A method according to any of the preceding claims in which the metal is aluminium and the bulk velocity of projected medium is from 15 to 100 meters per second.

6. A method according to any of the preceding claims including transporting the coil through succesive heat-treating stations pervaded by the gaseous medium while maintaining substantially steady state heat-treating conditions of each of the stations so as to subject the coil during transport to a predetermined heat-treating cycle.

7. A method according to claim 6 in which the coil is transported in a plurality of steps and remains stationary in each successive station for the same predetermined period of time.

8. A method of altering the temperature of a coil of metal strip substantially as described with reference to and as shown in the Figures.

9. Apparatus for heat-treating a coil of metal strip including means for supporting the coil and distribution means for projecting a temperature modifying gaseous medium upon an end of the coil so as to cause heat flow in the coil by conduction in the direction of the coil axis.

10. Apparatus according to claim 9 in which means are provided for projecting the medium in the form of jets.

11. Apparatus according to claim 9 or claim 10 in which the support means hold the coil so that the coil axis extends generally horizontally.

12. Apparatus according to claim 11 in which the distribution means arranged to project the medium upon both ends of the coil.

13. Apparatus according to any of claims 9 to 12 including means for transporting the coil through successive heat treating stations pervaded by the gaseous medium while maintaining substantially steady state heat-treating conditions of each of the stations so as to subject the coil during transport to a predetermined heat treating cycle.

14. Apparatus according to claim 13 in which the transporting means is arranged to to transport the coil in a plurality of steps with the coil remaining stationary at each successive station for the same predetermined period of time.

15. Apparatus for heat-treating coils of metal said apparatus having an enciosable annular furnace chamber with stationary sidewalls and roof and a rotatable annular hearth in which a plurality of separate distribution means is provided within the chamber for projecting a temperature-modifying gaseous medium against ends of the coils, the distribution means being positioned adjacent the inner and outer sidewalls respectively to form stations, and means movable conjointly with the hearth for supporting said coils with their axes in substantially radial alignment at the stations.

16. Apparatus according to claim 15 in which at least some of the separate distribution means include heat exchanger means.

17. Apparatus according to claim 15 or claim 16 in which the separate distribution means at each station are connected at their upper ends with medium-circulating means for drawing off the gaseous medium from within the chamber and returning it to the distribution means.

18. Apparatus according to any of claims 15 to 17 in which the inside wall of each of the distribution means is formed with a plurality of apertures in a pattern conforming to the end face of the coil to be supported at that station.

19. Apparatus according to any of claims 15 to 19 further including a loading and discharging station having a positive sealing door in its outside wall and positive sealing doors between the loading and discharging station and its adjoining stations.

20. Apparatus according to any of claims 15 to 19 further including means for intermittently rotating the hearth.

21. Apparatus for heat treating coils of metal strip substantially as described with reference to and as shown in the Figures.