US2832709A - Method and apparatus for long-cycle continuous annealing of strip metal - Google Patents

Description

i 29,1958 T. SENDZIMIR 2,832,709

METHOD AND APPARATUS FOR LONG-CYCLE CONTINUOUS ANNEALING OF STRIP METAL FiledJuly 2, 1956 IN VEN TOR. 750E052 SEWOz/M MWW METHGD AND APPTUS FOR LONG-CYCLE CONTWUOUS AYNEALING F STRIP METAL Tadeusz Sendzimir, Middletown, Ohio Application July 2, 1956, Serial No. 595,286

13 Claims. (Cl. 148-13) The invention relates to the continuous or strand annealmg of strip. Such operations have in the past been highly developed and widely used, having a number of advantages over box or batch annealing, the most important being a greater uniformity of product.

However, continuous or strand annealing is necessarily an operation of short duration, such as two or three minutes for a complete cycle, as compared with several hundred minutes in the case of batch annealing. As a consequence, it has not been possible to duplicate in continuous annealing operations the results which can be secured by box annealing. For example, in annealing low carbon steel strip continuously, although the temperatures may be equivalent to those attained in box annealing and may be applied more uniformly, the soaking period at temperature is necessarily short, and the resulting structure of the steel is more fine grained than that attained in a box anneal. While the physical properties and especially the elongation are excellent, the yield point is somewhat higher, resulting in. a stiffer sheet. This has some disadvantages in certain forming operations.

If in continuous furnaces any great production is to be attained, as for exampie to tons per hour, or speeds of hundreds of feet per minute, the furnace has to be made very long in order to insure that the rapidly moving strip comes up to temperature before it is cooled. Production furnaces have already attained practical limits of length; and to attempt to elongate them further would not only be impracticable in most instances, but would tremendously increase the cost wtihout providing any really useful extension of the soaking time. It has been suggested that the strip be looped in furnace sections over suitable rolls; but this greatly increases the transverse dimension of the furnace and its cost without providing the desired increase in the duration of the cycle. Moreover, it enormously complicates threading problems upon starting, or if a break should occur during operations.

it is an obiect of the invention to provide means and method for very greatly increasing the duration of a continuous anneali g cycle without proportionately increasing the size and cost of the furnace. Specifically, it is an object of the invention to provide means and a method whereby in a furnace section of relatively short length, so strip may be accumulated under high temperature conditions as to give a holding cycle of at least about one-quarter hour to one hour or longer.

It is an object of the invention to provide means and a method for the accumulation of a very great length of strip in continuous furnace which is not only relatively short, but also has a relatively small transverse dimension.

it is an object of the invention to provide for the accumulation of a very great quantity of strip in a relatively small space with a minimum of apparatus and with very greatly simplified threading.

It is an object of the invention to provide for the ac cumulation of strip in a small space in such fashion that States Patent Lit Patented Apr. 29, 1958 'ice , in the art upon reading these specifications, are accomplished in that procedure and by that construction and arrangement of parts of which certain exemplary embodiments are hereinafter described. Reference is made to the accompanying drawings wherein:

Fig. l is a diagrammatic vertical section of a furnace arrangement embodying the principles of the invention.

Figs. 2 and 3 are similar views of modifications thereof.

The invention is useful wherever continuous annealing may be employed; and while it is especially useful in annealing operations which are carried on in protective atmospheres, this is not a limitation. A particular field of utility in which continuous annealing is in widespread use today is in the coating of strip by immersion with molten coating metals in accordance with the teachings of my Patents 2,116,893, 2,136,957 and 2,197,622. The invention will herein be described in such an exemplary usage, it being understood that it is not limited thereto excepting as set forth in the appended claims.

Briefly, in the practice of the invention, the strip entering a heated furnace space is caused to descend therein for a substantial distance under the influence of gravity from a point of support (which may be the point of entry, a turning roll, or a pinch roll combination) until it reaches a transversely moving support. By reason of the gravitational descent of the strip, it forms on the moving support an accumulation of the strip in a series of configurations which are most conveniently described as accordion-like. in other words, the lower end of the gravitationally descending strip swings back and forth at a substantially regular frequency in a direction transverse the width of the strip, depositing on the moving support a series of folds or loops which overlap each other and are substantially of regular dimensions. The term fold as used herein is not necessarily employed to imply that the strip is creased back on itself sharply. Instead, the configurations formed on the support may be characterized by looped or rounded edges, as illustrated in the drawings hereof. The formation of the folds is accomplished without marring the strip since it is not caused to slide on itself or on the moving support. The depth of the configurations is determined by such factors as the distance between the point at which the gravitational descent begins and the moving support, the relative speeds of the strip and the moving support, the gauge and stiffness of the strip, and the like, all of which have an effect on the natural periodicity of swing of the lower end of the strip.

With folds of a given depth, the length of the transversely moving support or conveyor will determine how much strip can be accumulated thereon. Accumulated strip is continuously moved away from the point or area of accumulation on the moving support or conveyor and is continuously removed therefrom at the opposite end of the accumulation. It has been found that this may be accomplished equally without sliding, scratching, or marring, providing the accumulation is turned upsidedown within the furnace space so that the strip may be Withdrawn from the initial bottom of the accumulation. To this end the elongated accumulation is carried by the moving support or conveyor around a reversing means, as hereinafter described. When the accumulation has been released by the moving support and is resting on the reversing means, the strip may be withdrawn upwardly therefrom over a roll or pinch roll combination and removed from the furnace.

In Fig. 1 there is shown a strip 1, for example, cold rolled steel, being withdrawn by a pinch roll combination 2 from a coil 3. Since the apparatus is designed to operate continuously, provision is made for joining the ends of successive strips. The index numeral 4 indicates apparatus for this purpose, which may comprise a shear and welder, or other suitable means, followed by a pinch roll combination 5. In order that the joining of strip ends will not interrupt the continuous passage of the strip into the furnace, a looper, diagrammatically indicated at 6, will be included in the line. The strip is passed into a heating chamber 7 through pinch rolls 8 having sufiicient drag to provide tension in the heating chamber. The heating chamber may be horizontal, vertical, or inclined.

In many instances a pretreatment will be desired for the strip before it enters the heating chamber. Thus it may be passed through an oxidizing furnace to burn oils and greases from its surfaces and to produce on said surfaces a thin, controlled film of oxide. Or the strip may be subjected to chemical cleaning, wetting and drying and other pretreatments, all in accordance with my patents hereinabove recited, the disclosures of which are incorporated herein by reference. Devices for such pretreatments have not been included in the present drawings, excepting to indicate that it is possible to subdivide the heating chamber 7 by means of a partition or seal 9 into an oxidizing portion 7a and a reducing portion 71;. The oxidizing portion will normally be heated by open-flame burners having slightly oxidizing products of combustion, while the reducing portion may be heated electrically or by radiant tubes, and may contain a reducing atmosphere. It is preferable, though not necessary, that the strip be brought up to the desired high temperature before it enters the next furnace section 10. The furnace section 7 may be provided with fast heating units having retarded oxidation characteristics so as to feed the strip 1 into the holding furnace section 10 with a submicroscopic, instantly reducible layer of oxide on it. The strip is fed into the furnace section 10 by pinch rolls 11, and is passed over a roll 12, descending freely by gravity therebeyond toward a moving support or conveyor 13.

The conveyor 13 extends substantially the whole length of the furnace section 10, the far end of the conveyor being upwardly and reversely curved, as later set forth. The conveyor may be made up of closely spaced, heatresistant, driven rollers, interdigitated or not, or it may be a heatresistant, driven chain conveyor, supported by rollers which in that case do not need to be so closely spaced. The conveyor 13 is driven at a speed which is only a small fraction of the linear speed of the strip 1. if the conveyor were stationary, the freely falling strip would keep forming folds on it in vertically stacked relationship. Since the conveyor is driven in the direction of the arrow, the folds of the strip are more or less offset, and the accumulated strip travels along the conveyor. The height of the accumulation, i. e. the number of superposed folds in it, remains constant as long as the ratio between the speed of the strip and the speed of the conveyor remains constant.

Thus, by way of example, if the speed of the conveyor is feet per minute, while the speed of the strip is 500 feet per minute, this is a ratio of 1:109; and the accumulation of strip on the conveyor will be 50 folds in transverse depth. This is the equivalent of 100 thicknesses of the strip; and if the length of the conveyor be 500 feet..the time spent by any given portion of the strip in the furnace section is 100 minutes. This is a length of time fully sufiicient for an adequate soaking period, after which the strip may be cooled in a single strand 'without'damage to the grain structure of the steel. The

figures given above are, of course, illustrative, and do not constitute a limitation on the invention.

As hereinabove set forth, the conveyor 13 is curved upwardly and reversed near the exit end of the furnace section 10. This is indicated at 130. A reversing con veyor is provided cooperating with the reversed portion of the conveyor 13 and serving therebeyond to support the accumulation 1a of strip in the upside-down position. The reversing conveyor may comprise chains 14, passing over sprockets 15 and 16, one at least of which is driven, the upper flight of the chains being supported by spaced rollers or sprockets 17. The strip accumulation in is engaged between the two conveyors, the slantwise disposition of the second conveyor serving to compress the folds slightly (without necessarily forming creases in the metal); and the accumulation is turned upside-down as shown. After the reversal, the conveyor 13 terminates, and the accumulation rests freely on the chains 14 so that the strip may be withdrawn upwardly from it by a simple unfolding operation not involving sliding or slipping and consequent scratching. The upwardly traveling strip passes over a turning roll 18, and then is withdrawn from the furnace section 10 to a cooling section.

The furnace section 10 may have any suitable construction, and may be heated in any suitable way consistent with the maintenance in it of any atmosphere desired. This is usually a reducing atmosphere. Radiant tube heaters have been diagrammatically indicated at 19, but other means may be employed. The shape of the furnace may be varied, and can be caused to conform quite closely to the apparatus and accumulation therein, keeping in mind that provision must be made for vertical travel of the strip as it enters and as it leaves the accumulation 1a. The temperatures reached in the furnace can be selected as necessary for the effects desired. In securing effects comparable to those of a box anneal, cold rolled steel will be heated to a temperature above the recrystallization point, say, 1750 F by way of example. The furnace section 10 is one in which the strip is to be maintained at' a desired temperature for a desired soaking period. The temperature of the strip may be maintained constant throughout the furnace section 10; but it is possible in many instances to effect an appreciable slow lowering of the temperature of the strip While still folded, near the exit end of the furnace section, say to a temperature below the recrystallization point. The length of the furnace section may be varied within any practicable limits. The conformation of the strip in the accumulation 1a facilitates uniform temperature maintenance.

The strip will, in the case of bright annealing or coating, be carried to a cooling chamber 20 for further cooling or rapid cooling in a non-oxidizing atmosphere to a temperature which will depend on the kind of final product that is to be made. The cooling chamber may take various forms. It may simply be an elongated cooling chamber as known in the art supplied with non-oxidizing gas, or it may be a shorter chamber in which the strip is looped for cooling purposes. If the purpose is bright annealing, the chamber may be arranged to cool the strip down to such a temperature that it may be withdrawn into air without danger of oxidation. In the event of coating, the reduction in temperature will preferably be to a point slightly above the temperature of the molten coating metal.

In Fig. l the strip is drawn into the cooling chamber 20 by a pinch roll combination 21 and is looped therein as at 1b. Cooling means, such as a series of internally fluid cooled tubes 22, may be employed in the cooling chamber.

In the particular embodiment the strip enters a metal coating bath 23 through a hood 24 which dips into the bath to form a seal. The bath is maintained in a pot 25; and the heat of the strip may be wholly or partially relied upon to maintain the coating metal molten in the bath.

nose r09 A non-oxidizing or reducing atmosphere may be introduced into the apparatus through means 26 and may be caused to protect the metai throughout the hood, the cooling chamber, the annealing chamber 10, and the heating furnace 7.

The strip turns around a roller 27 submerged in the molten coating metal and is Withdrawn upwardly from the bath through exit rolls 28 in instances where these may be used. The treated strip is carried upwardly a sufficient distance to insure complete freezing of the coating metal on its surfaces and then may be looped as diagrammatically illustrated at llc until its temperature has been lowered sufficiently to enable it to be coiled as at 29, cut into sheets, or otherwise treated. In particular, prior to the final coiling, it may be desired to subject the coated metal to the means shown and described in the copending application in the name of the same inventor entitled The Prevention of Coil Breaks, Serial No. 595,224, filed July 2, 1956. Additional treatments may be practiced as dictated by the nature of the product. It will be understood that where the temperature of the strip can be sufficiently lowered after suitable soaking in the furnace section it), a cooling chamber as such may be eliminated.

A modification is illustrated in Fig. 2 wherein in the furnace section It the accumulation 1a is made, as hereinabove described, on the moving conveyor 13. At a point adjacent its point of formation, the accumulation w is engaged by a horizontal conveyor comprising, for example, chains 30 moving over sprockets 31 and 32. The lower flight of the chains is supported by spaced rollers 33 and its upper flight by spaced rollers 34.

The conveyor just described is greatly elongated. The conveyor 13 curves about it at the far end as indicated at 13a and then terminates, so that the reversed accumulation rides back over the top of the conveyor comprising the chains 34), as indicated at 1d. This efiectively increases the length of the accumulation in any given length of the furnace section 10. The strip may be withdrawn from the accumulation over the roller 13 at a point near the entrance of the furnace section 19. Radiant tube heaters are diagrammatically indicated at 35 and 36. The arrangement shown in Fig. 2 makes for additional uniformity in the treatment of the strip. The conveyor comprising the chains 30 may be driven if desired. In many instances, however, it can be frictionally driven by engagement with the accumulation in.

Yet another arrangement is shown in Fig. 3 where within the furnace section 10 the accumulation 1a of the strip 1 is formed, as previously described, on a transversely moving conveyor 13. The conveyor curves about the periphery of a large diameter roll or drum 37 so as to carry the accumulation lla upwardly to a position 1e where it is released by the conveyor 13 and overlies the top of drum 37, whence it may be withdrawn, as previously described, over the roller 18. The drum 37 may be driven independently or by the accumulation in.

The arrangement shown in Fig. 3 does not provide for as great a length of the accumulation of strip as do arrangements in which the conveyor 13 is more elongated; but a plurality of the arrangements shown in Fig. 3 may be included in the same or in contiguous heating or soaking furnace chambers.

By any of the arrangements shown, the soaking cycle in a continuous annealing furnace may be very greatly prolonged without marring the strip and without subjecting it to undue tension.

Modifications may be made in the invention without departing from the spirit of it. The invention having been described in certain exemplary embodiments, what is claimed as new and desired to besecured by Letters Patent is:

l. A process of continuously annealing metallic strip material which comprises feeding a continuous length of strip in a single strand and continuously bringing its temperature up to a desired annealing temperature, followed by causing said strip to drop freely onto a transversely moving conveyor in a soaking zone, the said conveyor moving at a substantially slower speed than the speed of the said strip, thus causing an accordion-like accumulation of folds of the strip to be formed on said conveyor in offset relationship and moved away from the point of formation as an accumulation, subjecting said accumulation to soaking temperatures, and then turning the end of said accumulation opposite the point of formation upsidedown and withdrawing the strip in a single strand upwardly from the said accumulation.

2. The process claimed in claim 1 in which the said steps are performed in a bright annealing atmosphere and in which the said strip is cooled from the soaking temperature to a non-oxidizing temperature prior to withdrawal to the outer air.

3. The process claimed in claim 1 in which the said steps are performed in a bright annealing atmosphere and in which the said strip is cooled from the soaking temperature to a non-oxidizing temperature prior to withdrawal to the outer air, wherein the strip is a ferrous strip, and wherein the said strip is soaked at a temperature above the recrystallization temperature in said accumulation for a period of time of at least about onequarter hour.

4. The process claimed in claim 1 wherein the steps are performed in a reducing atmosphere and in which the said strip is passed while still protected by said atmos phere into a bath of molten coating metal.

5. The process claimed in claim 1 wherein the said steps are performed in a reducing atmosphere and in which the said strip is passed while still protected by said atmosphere into a bath of molten coating metal, and in which said strip is cooled prior to its passage into the molten coating metal to a temperature approximating that of the molten coating metal.

6. The process claimed in claim 5 wherein the said strip is a ferrous strip and in which it is soaked to a temperature above the recrystallization temperature for a period of time at least about one-quarter hour.

7. The process claimed in claim 6 wherein the said strip is cold rolled strip, wherein its surfaces are freed of oils and greases, and wherein a very thin, reducible oxide coating on the surfaces of said strip is reduced in the said heating process.

8. The process claimed in claim 7 wherein the surfaces of the strip are freed of oils and greases by heating in an oxidizing atmosphere.

9. The process claimed in claim 7 wherein the surfaces of the strip are freed of oils and greases by chemical cleaning, including wetting and drying.

10. In apparatus for the purpose described, a heat treating furnace chamber, a substantially horizontal conveyor in said chamber, driving means for continuously introducing a strip into said chamber and for causing it to drop freely onto an end portion of said conveyor so as to form loose folds thereon, means for driving said conveyor at a speed substantially less than the speed of travel of said strip whereby said folds are formed in offset position, constituting an accumulation of said strip in loose folds which moves with said conveyor, the said conveyor curving upwardly and reversely at its far end over a turning means so that the said accumulation is turned upside-down above said turning means While still in the form of loose folds, and means for withdrawing the strip as a single strand upwardly from the upside-down portion of said accumulation.

11. The structure claimed in claim 10 wherein said turning means is a large diameter drum. 7

12. The structure claimed in claim 10 wherein said turning means is a slantwise disposed conveyor of relatively short length as compared with the length of said first mentioned conveyor.

13. The structure claimed in claim 10 whereinsaid said turning means is a substantially horizontally disposed conveyor overlying the first conveyor and extending for a substantial distance in the reverse direction.

14. The structure claimed in claim 10 in combination with a preceding furnace section for heating the strip as a single strand to an annealing temperature.

15. The structure claimed in claim 10 in combination with a preceding furnace section for heating the strip as a single strand to an annealing temperature, and including a cooling section following the first mentioned heating section.

16. Apparatus as claimed in claim 15 including means for maintaining throughout said sections a non-oxidizing protective atmosphere.

17. Apparatus as claimed in claim 15 including means for maintaining in said sections a reducing atmosphere and including a pot of molten coating metal to which said O (L! strip is delivered by means of a hood from said cooling section without re-exposure to air.

18. Apparatus as claimed in claim 17 including initial means for freeing the surfaces of said strip from oils and greases.

References Cited in the file of this patent UNITED STATES PATENTS 2,180,601 Mitchell et al. Nov. 21, 1939 2,224,410 Sendzimir Dec. 10, 1940 2,325,544 Redman July 27, 1943 2,431,372 Cook et al Nov. 25, 1947 FOREIGN PATENTS 888,103 France Aug. 30, 1943

Claims (1)

1. A PROCESS OF CONTINUOUSLY ANNEALING METALLIC STRIP MATERIAL WHICH COMPRISES FEEDING A CONTINUOUS LENGTH OF STRIP IN A SINGLE STRAND AND CONTINUOUSLY BRINGING ITS TEMPERATURE UP TO A DESIRED ANNEALING TEMPERATURE, FOLLOWED BY CAUSING SAID STRIP TO DROP FREELY ONTO A TRANSVERSELY MOVING CONVEYOR IN A SOAKING ZONE, THE SAID CONVEYOR MOVING AT A SUBSTANTIALLY SLOWER SPEED THAN THE SPEED OF THE SAID STRIP, THUS CAUSING AN ACCORDION-LIKE ACCUMULATION OF FOLDS OF THE STRIP TO BE FORMED ON SAID CONVEYOR IN OFFSET RELATIONSHIP AND MOVED AWAY FROM THE POINT OF

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