GB2048959A - Finishing Method and Apparatus for Conventional Hot Dip Coating of a Ferrous Base Metal Strip With a Molten Coating Metal - Google Patents

Abstract

A finishing method and apparatus for conventional continuous hot-dip coating of the type wherein a ferrous base metal strip is caused to pass beneath the surface of a coating bath of molten coating metal and is thereafter subjected to jet finishing, the ferrous base metal strip having been appropriately pretreated so as to be at the proper coating temperature and so as to have its surfaces oxide- free when passing through the bath of molten coating metal. The method (see Figure 2) comprises the steps of providing an enclosure for the two-side coated strip (9) as it exits the coating bath (7), locating a finishing jet nozzle (28, 29) to either side of the coated strip within the enclosure, jet finishing the coated strip with a non-oxidizing or inert gas and maintaining the jet finishing gas and the atmosphere within the enclosure at an oxygen level of less than about 200 parts per million. The apparatus comprises the above mentioned enclosure with the jet finishing nozzles located therein and an appropriate system (34) which provides a non-oxidizing or inert atmosphere within the enclosure. The non-oxidizing atmosphere may be nitrogen. <IMAGE>

Description

SPECIFICATION Finishing Method and Apparatus for Conventional Hot Dip Coating of a Ferrous Base Metal Strip with a Molten Coating Metal This invention relates to a finishing method and apparatus for conventional continuous hot-dip coating of a ferrous base metal strip with a molten coating metal, and more particularly to a method and apparatus whereby the coated ferrous base metal strip, upon exiting the coating bath, is maintained in an essentially oxygen-free atmosphere until jet finished with a non-oxidizing or inert gas.

The method and the apparatus of the present invention are applicable to the hot-dip coating of a ferrous base metal strip with zinc, zinc alloys, aluminum, aluminum alloys, terne, lead and those coating metals or coating metal alloys which have oxide forming characteristics such that acceptable finishing cannot be accomplished by conventional jet practice or by conventional exit rolls. While not intended to be so limited, for purposes of an exemplary showing the method of the present invention will be described as applied to galvanizing. The method can be practiced on various types of galvanizing lines.For example, the method of the present invention is applicable tofluxless, hot-dip metallic coating of ferrous base metal strip wherein it is necessary to subject the strip surfaces to a preliminary treatment which provides the strip with oxide free surfaces and preferably brings the strip.to a temperature approximating that of the molten zinc or zinc alloy coating bath at the time the strip is caused to pass beneath the surface thereof. One of the principal types of anneal-in-line, fluxless, preliminary treatments is the so-called Sendzimir process or oxidation-reduction practice disclosed in U.S. Patents 2,110,893 and 2,197,622.

Another anneal-in-line, fluxless preliminary treatment in common use is the so-called Selas process or high intensity direct fired furnace practice disclosed in U.S. Patent 3,320,085.

In the Sendzimir process a ferrous base metal strip is heated in an oxidizing furnace (which may be a direct fired furnace) to a temperature of about 3700C to about 48500 without atmosphere control, withdrawn into air to form a controlled surface oxide layer layer varying in appearance from light yellow to purple or even blue, introduced into a reduction furnace containing a hydrogen and nitrogen atmosphere wherein the stock is heated to about 7350C to about 925or and the controlled oxide layer is completely reduced. The stock is then passed into a cooling section containing a protective reducing atmosphere, such as a hydrogen-nitrogen mixture, brought approximately to a temperature of the-molten coating metal bath, and then is led beneath the bath surface while still surrounded by the protective atmosphere.

In the Selas process the ferrous base metal strip is passed through a direct fired preheat furnace section. The strip is heated by direct combustion of fuel and air producing gaseous products of combustion containing at least about 3% combustibles in the form of carbon monoxide -and hydrogen. The strip reaches a temperature of about 5350C to about 7600C while maintaining bright surfaces completely free of oxidation. The strip is then passed into a reducing section which is in sealed relation to the preheat section and which contains a hydrogen and nitrogen atmosphere, wherein it may be further heated by radiant tubes to about 6500C to about 9250C and thereafter cooled approximately to the molten coating metal bath temperature. The strip is then led beneath the bath surface while surrounded by the protective atmosphere.

Other related pretreatment techniques are taught in U.S. Patents Re 29,726-3,837,790- 4,123,291-4,123,292 and 4,140,552. The above mentioned prior art patents constitute nonlimiting examples of fluxiess, continuous galvanizing processes to which the method of the present invention is applicable. When such conventional strip preparation techniques as are taught in the above mentioned prior art are used, it is necessary that the base metal strip be maintained in a protective atmosphere at least until it passes beneath the surface of the bath of molten zinc or zinc alloy.

Such a protective atmosphere is not a requirement when flux or chemical strip preparation techniques of the type taught in United States Patents 2,824i028 and 2,824,021 are employed. Briefly, when such chemical strip preparation techniques are used, the ferrous base metal strip is caused to pass through a flux bath and through means to assure the proper thickness of the flux coating on the strip. The ferrous base metal strip is then conducted through a heating chamber wherein the strip is heated to evaporate the water in the flux solution. Thereafter, the ferrous base metal strip is further heated to raise it to that temperature approaching the maximum temperature of stability of flux coating on the strip. The strip is then caused to pass beneath the surface of the bath of molten zinc or zinc alloy so as to be coated.The method of the present invention is equally applicable to galvanizing lines utilizing such flux or chemical pretreatment systems.

From the above it will be evident that the method of the present invention is not limited to the use of any particular pretreatment of the ferrous base metal strip in the galvanizing line and the terms "pretreatment" or "pretreated" (as used herein and in the claims with reference to the ferrous base metal strip) are to be interpreted broadly to include any of the conventional pretreatment systems exemplified by the above noted prior art. In general, these terms refer to any appropriate pretreatment technique, the result of which is such that, during the actual coating step wherein the ferrous base metal strip passes through the molten bath of zinc or zinc alloy, it will be at or will achieve the proper coating temperature and its surfaces will be oxide-free.

In conventional continuous hot dip galvanizing, it is usual to cause the two-side coated strip to exit the molten coating metal bath into the ambient atmosphere. The most widely used finishing and coating weight control technique is to direct the coated strip between jet knives or nozzles which cause a blast of air or steam to impinge upon both sides of the coated strip, returning excess coating metal to the bath. This finishing technique, however, has a number of definite drawbacks. One drawback is the formation of dross at the surface of the molten coating metal bath. The formation of such dross represents considerable loss of zinc values and some of the surface dross is often drawn up by the coated strip, passing through the jet blast, and forming visible surface dross iump defects in the coated product.

Another common coating defect or nonuniformity related to conventional jet finishing is frequently referred to as "coating ripples" or "ocean waves". Coating ripples can essentially be described as wave-like nonuniformities in coating thickness in the longitudinal (rolling) direction of the coated strip. Coating ripples can vary in severity from essentially none to very heavy ripples which are often called "coating sags". It is very difficult to completely eliminate coating ripples in conventional jet finishing practice and nearly impossible at speeds below about 45.5 meters per minute. High speed, close positioning of the jet nozzles to the strip, high aluminum content in the zinc bath and minimum coating weight are means employed to reduce coating ripple severity in conventional practice.

Yet another hot-dip zinc coating from nonuniformity is commonly referred to as "spangle reiief". Spangle relief has two aspects. One is the variation of surface profile (zinc thickness) across the zinc crystal from one boundary to the opposite boundary. The other is a depressed spangle boundary which surrounds each spangle or crystal. Both aspects are related to the dendritic solidification characteristics of zinc coatings.

Spangle relief can be reduced by such methods as purposely causing part of the zinc coating to alloy with the ferrous base metal, by decreasing the lead content of the zinc bath, or by antimony additions to the zinc bath. However, none of these methods is entirely satisfactory: As a result, many methods have been developed to suppress spangle formation. That is, to minimize final spangle size to such an extent that the spangles are hardly visible to the naked eye. For example, United States Patents 3,379,557 and 3,756,844 teach spangle minimizing methods. Most methods involve spraying water or water solutions against the molten coating to quench the coating and create many nucleation sites.

While spangle minimizing methods are effective in minimizing spangle relief, such methods suffer operating and maintenance shortcomings when considered on a routine basis and the results achieved by them are not alway consistent.

Spangle minimizing methods do nothing to overcome jet finishing ripples and coating dross.

The various hot dip coating non-uniformities present in conventionally jet finished zinc coatings can be masked by temper (skin-pass) rolling.

However, temper rolling causes the nonuniformities to become imprinted in the base metal. As a result of this non-uniform cold working of the base metal, the defects may reappear when critical surface items such as automotive body parts are stamped or formed.

Another major problem area encountered with conventional jet finishing is that of coating control at the edges of the strip. One edge problem is that of zinc coating thickness over a narrow band immediately adjacent each edge of the coated strip. The coating thickness of these bands is greater than the coating thickness over the rest of the strip width. If this coating thickness differential is great enough, edge build-up or spooling will occur when the continuous strip is coiled under tension.

Other troublesome problems include edge berries (small balls of oxide) which are attached to the strip edge and are pulled though the jet blast.

Furthermore, an edge defect commonly known as "feathered oxide" occurs during low speed jet finishing. Feathered oxide is characterized by discontinuous patches of heavy coating metal oxide which pull through the jet blast. They appear much like feathers which extend inwardly from the strip edges with the tips thereof pointing toward the center of the strip.

Many methods have been used by prior art workers to reduce build-up and oxide control problems at the strip edges. Tapered jet nozzle slot openings are commonly used where the slot opening of the jet finishing nozzle continuously increases in width from the center of jet nozzles to its ends. Such a contoured jet finishing nozzle is taught in United States Patent 4,137,347.

Other methods to control edge coating include curving the jet nozzles so that the nozzle is closer to the strip at the strip edges than at the strip center Also, vanes or nozzle extensions have been used at the strip edges to bring the nozzle closer to the edges than to the center of the strip.

Still other methods include the use of shutters and auxiliary jets, bothnternal and external to the main jet nozzles, to alter the jet wiping force at the strip edges as compared to the jet wiping force at the strip center.

All prior art methods fall short of producing optimum edge control with a minimum of operator attention, maximum coating metal economy, sufficient edge control for low speed operation and proper edge control over a wide range of strip widths.

Yet another problem area encountered with conventional jet finishing involves coating weights and line speeds. The viscous interaction between the coating metal and the strip is proportional to strip speed. At slow speeds, the prior art was faced with the problem of ripple formation. To combat this, it was found that reducing the jet finishing flow rate will break up the oxide and more evenly distribute it. However, low jet finishing pressure and close positioning of the jet finishing nozzles at the time creates an edge build-up problem. Prior art workers therefore have has to adjust the parameters to control edge build-up and ripples and this has necessitated higher line speeds. As an example, it has been common practice to use conventional jet finishing only with strip speeds above 30 meters per minute to produce commercial class coating weight (ASTM A525, G-90).Edge build-up problems on G-90 coating (coating weight of 275 g/m2) commonly occur at speeds below about 45.5 meters per minute. Minimum operating speed for heavier coatings, such as 564 g/m2 (G185), are even more restrictive and edge-to-edge coating uniformity deteriorates with increasing coating weight.

Another essential practice in most prior art jet finishing operations is to position the jet nozzles virtually directly opposed such that the jet streams are in direct interference beyond the edges of the strip. This interference results in extremely high and objectionable noise levels. If the jet nozzles are operated while vertically offset with respect to each other, a wraparound effect can result whereby the last jet to operate on the strip causes a bead of heavy coating metal to form along the edge on the opposite side of the strip. In addition to the noise problem and the need for precise adjustment of the jet nozzles by the operator, opposed operation can result in coating metal splatter being blown off of the strip edge by one nozzle and into the nozzle opening of the opposed nozzle.

Prior art workers have hitherto jet finished hotdipped, two-sided coated galvanized and aluminized strip with nitrogen. Such jet finishing, however, has been performed in an ambient atmosphere. In jet finishing, less nitrogen is required than air. However, the results achieved by such finishing are most nearly like those achieved in jet finishing with air in an ambient atmosphere than like the results achieved by the present method.

U.S. Patents 4,107,357 and 4,114,563 and German Patent 2,656,535 are exemplary of patents teaching methods for coating one side only of a ferrous base metal strip. In the practice of these processes, the coated strip after contact with the coating bath is maintained in a protective, non-oxidizing atmosphere and is jet finished with nitrogen or a non-oxidizing gas.

However, the primary purposes of these steps is to prevent oxidation of that side of the ferrous base metal strip not coated or, if the uncoated side has an oxide film thereon, to prevent adherence of the coating metal to the oxide film.

The present invention is based upon the discovery that if, in a conventional, continuous, hot-dip, two-side galvanizing process, the coated metal as it exits the coating bath is surrounded by an enclosure in which a substantially oxygen-free atmosphere is maintained and if within the enclosure the coated strip is jet finished with a non-oxidizing or inert gas, the finishing problems encountered with conventional finishing methods are markedly reduced or eliminated. The finishing method of the present invention produces extremely flat spangle with so little spangle boundary relief that it is no longer necessary to practice spangle minimizing techniques to achieve excellent surface quality. Dross is greatly reduced together with dross-related problems and jet finishing ripples are eliminated, even at slow operating speeds.With the marked reduction in dross formation, loss of zinc values to top skimmings is greatly reduced.

One of the most significant aspects of the present invention is the discovery that all coating control problems at the strip edges are completely eliminated with the exclusion of oxygen from the finishing process. Minimum operating speeds are no longer limited by edge build-up problems, but rather only by the desired coating weight relative to the amount of coating metal naturally pulled up by the strip to the finishing jet nozzles. It has been found, for example, that excellent quality coatings of 275 g/m2 can be produced without difficulty at speeds as low as 9.1 meters per minute. Edge wrap around does not occur, so that the jet nozzles can be vertically offset, eliminating the need for precise positioning, greatly reducing noise, and eliminating the zinc splattering hazard.

Jet nozzle design may be simplified to utilize a nozzle having a slot-like nozzle opening of uniform width throughout its length, doing away with the multitude of special jet nozzle designs, methods and accessories which have been used for controlling edge build-up. Superior uniform coating results edge-to-edge for all coating weights because the center profile need no longer be distorted to compensate for heavy edges.

Heretofore, in the practice of conventional twoside jet finishing, neither the mechanism of ripple formation nor that causing edge build-up problems was completely understood. In the process of conventional jet finishing, a pneumatic dam effect is created whereby the desired amount of coating metal is metered through the jet barrier to form the finished coating. At this metering point the excess coating metal pulled up with the strip, beyond that required for the finished coating, is returned to the coating bath. This process is described in detail in United States Patent 4,078,103.

While applicants do not wish to be bound by theory, it appears as a result of the present invention that the coating ripples and heavy edge coating in conventional jet finishing are caused entirely by coating metal oxide. At some point in the jet interaction region, probably just above the point of zero surface velocity, fresh (unoxidized) coating metal is being exposed and, as it is exposed, it immediately forms a very light oxide skin. The continuity of flow or distribution of this very light oxide skin onto the finished coating determines the occurrence of coating ripples. In conventional practice, the jet periodically restrains the oxide film. The film builds up until the jet no longer can restrain it. At this time a segment of relatively heavy oxide breaks off and passes with the finished coating.The segment as it passes carries with it coating beneath which is heavier than that which is metered on when the oxide film is restrained. This process is repeated many times each second as ripples are formed.

A similar mechanism is believed to be operable in creating heavy coating metal along the strip edges. However, at the edges, geometry becomes an additional important factor in that there is no wiping force directed against the edge surfaces.

Relatively heavy oxide is permitted to pass through the jet interaction area more or less continuously carrying with it heavy coating beneath. This oxide envelope around each strip edge surface is the "container" which permits zinc wraparound to occur when the jet nozzles are vertically offset.

These coating nonuniformities are caused they molten coating metal andare eliminated in the present invention by avoiding oxidation.

The zinc coated product produced by the method of the present invention has such excellent surface qualities after temper rolling that it is suitable for use in exposed automotive body panels, appliance applications and the like. The practice of the present invention lends itself well to short immersion, shallow coating pot practice utilizing a partially submerged pot roll.

According to the invention there is provided a finishing process for continuous hot-dip, two-side coating of a ferrous base metal strip with a molten coating metal of the type wherein said ferrous base metal strip is caused to enter a bath of said molten coating metal contained in a coating pot, said ferrous base metal strip having been treated to bring it to a coating temperature sufficiently high to prevent casting of said coating metal thereon and low enough to prevent excess coating metal-base metal alloying and to render the surfaces of said strip clean and free of oxide as it passes through said molten coating metal bath, characterized by the steps of providing an enclosure in sealed relationship with said bath for said two-side coated ferrous base metal strip as it exists said bath and an exit in said enclosure for said coated strip, maintaining a non-oxidizing atmosphere within said enclosure, locating a jet finishing nozzle to either side of said coated strip within said enclosure, jet finishing said coated strip with a non-oxidizing gas and maintaining said jet finishing gas and said atmosphere within said enclosure at an oxygen level of less than about 220 ppm, whereby to render said two-side coated strip free of oxide-caused coating nonuniformities, Finishing apparatus in accordance with the present invention adapted to practice of the above process is characterized by an enclosure for ferrous base metal strip as it exits a molten coating metal bath, the enclosure having an open bottom and extending into the molten coating metal bath, the enclosure having an exit slot formed therein for said strip, a pair of jet finishing nozzles located within the enclosure, above the molten coating metal bath and to each side of the strip, means to maintain a non-oxidizing gas within the enclosure having an oxygen level of less than about 200 ppm, and means to supply the jet finishing nozzles with a non-oxidizing gas having an oxygen level of less than about 200 ppm.

Brief Description of the Drawings Figure 1 is a fragmentary, semi-diagrammatic, cross sectional elevational view of an exemplary continuous hot-dip galvanizing line equipped to practice the method of the present invention.

Figure 2 is an enlarged, fragmentary, semidiagrammatic, cross section view of the coating end of the galvanizing line of Figure 1.

Figure 3,4 and 5 are enlarged, fragmentary, semi-diagrammatic cross sectional views similar to Figure 2, but illustrating various pot roll arrangements.

Figure 6 is a fragmentary, semi-diagrammatic cross sectional view illustrating the enclosure of the present invention as constituting an integral part of the snout through which the strip enters the bath of molten coating metal.

Figure 7 is a fragmentary, semi-diagrammatic cross sectional view, similar to Figure 6, but showing a partially submerged pot roll and the use of a pump for the molten coating metal.

Detailed Description of the Invention While not intended to be so limited, for purposes of an exemplary-showing the method of the present invention will be described as applied to a Selas-type galvanizing line. Turning to Figure 1, the coating line is generally indicated at 1. The strip preparation furnace of the coating .ine comprises a direct fired furnace 2, a controlled atmosphere heating furnace 3, a first cooling section 4, a second cooling section 5 and a snout 6. It will be noted that the snout 6 is configured to extend below the upper surface of a bath 7 of molten coating zinc or zinc alloy, located in a coating pot 8.

The ferrous base metal strip 9 to be prepared enters the direct fired furnace 2 over rolls 1 0 and 11 and through sealing rolls 12 and 13, so located as to minimize the escape products of combustion through the entrance opening 14 of preheat furnace 2. The direct fired furnace 2 operates at a temperature on the order of 1 2600C. The function of the direct fired furnace is to quickly burn away oil and the like from the surfaces of the ferrous base metal strip 9, while providing partial heating for annealing the strip.

The direct fired furnace, at the temperature indicated, will be sufficient to heat the entering strip to a temperature of from about 5350C to about 7609C by the time it passes from the direct fired furnace to the controlled atmosphere heating furnace 3.

The ferrous base metal-strip 9 passes about turnaround rolls 1 5 and 1 6 and begins an upward travel through controlled atmosphere heating furnace 3. Thereafter, the strip passes about turnaround roll 1 7 and continues downwardly again through furnace 3. The controlled atmosphere heating furnace may be of the radiant tube type and will further raise the temperature of the ferrous base metal strip 9 to from about 6500C to about 9250C, depending upon the nature of the ferrous base metal strip and the desired final characteristics of the base metal strip.

The strip preparation furnace of the coating line 1 may have one or more cooling chambers.

For purposes of this exemplary showing, the strip preparation furnace is illustrated as having two cooling chambers 4 and 5. From the controlled atmosphere heating furnace 3 the strip 9 passes about turn-around rolls 1 8 and 19 and enters cooling chamber 4. Chamber 4 may be of the tube cooling type well known in the art. In the exemplary illustration, the ferrous strip 9 makes three vertical flights through cooling chamber 4, passing about turn-around rolls 20 and 21.

Thereafter, the ferrous hase metal strip 9 passes about turn-around rolls 22 and 23 to enter the second cooling chamber 5 which may be of the jet cooling type, again well known in the art.

The temperature to which the ferrous base metal strip 9 is cooled will depend upon a number of factors. Since the molten coating metal 7 in coating pot 8 is zinc or zinc alloy, the ferrous base metal strip will preferably be cooled to approximately 4500 C. In some instances, however, the strip itself may be used as an additional means to introduce heat into the molten coating metal bath 7. Under these circumstances the ferrous base metals strip 9 may be introduced into the bath 7 at a temperature somewhat higher than the melting point of the zinc or zinc alloy therein. Where the strip is not relied upon as one of the heat sources for the bath 7, the strip may be introduced into the bath at a temperature slightly below that of the bath. In any event, the strip temperature should be sufficiently high as to prevent casting of the molten coating metal thereon.By the same token, the strip temperature must not be so high as to bring about excess coating metal-base metal alloying.

From the cooling chamber 5 the ferrous base metal strip 9 passes about turn-around roll 24 and enters the snout 6. It will be noted that the free end of snout 6 extends downwardly below the surface of the zinc or zinc alloy bath 7. The ferrous base metal strip passes about turn-down roll 25 and is directed downwardly into bath 7.

Within the bath, the strip is guided by one or more coating pot rolls so as to exit in a substantially vertical flight. In the embodiment shown, a single coating pot roll 26 is illustrated. The two-side coated ferrous base metal strip 9a exits the molten coating metal bath 7 and enters an enclosure 27, the lower end of which extends into the molten coating metal bath 7 to form a seal therewith. Within enclosure 27, the two-sided coated ferrous base metal strip 9a is caused to pass between a pair of jet finishing nozzles 28 and 29.

It will be noted from Figure 1 that the upper end of the direct fired furnace 2 is connected by a conduit 30 to an exhaust fan 31. The outlet 32 of exhaust fan 31 may be connected directly to a stack or to waste gas heat reclamation means (not shown). The strip preparation furnace of coating line 1 can be operated above atmospheric pressure (to prevent the introduction therein of oxygen from the ambient atmosphere) by controlling the discharge rate of the products of combustion from the directed fired furnace 2. To this end, a damper 33 may be located in conduit 30. The parameters under which the strip preparation furnace of coating line 1 is run do not constitute a limitation on the present invention.

Reference is now made to Figure 2 wherein the snout 6, coating pot 8 and enclosure 27 of Figure 1 are shown enlarged. Like parts have been given like index numerals. In the embodiment of Figures -1 and 2, snout 6 and the enclosure 27 are illustrated as constituting wholly separate structures. It will be understood by one skilled in the art that the enclosure 27 could constitute an integral part of snout 6. When a chemical and a flux pretreatment system is used, the snout 6 can be eliminated.

In accordance with the method of the present invention, a non-oxidizing atmosphere is maintained within enclosure 27 having an oxygen content of less than about 200 ppm, and preferably less than about 100 ppm. Any appropriate non-oxidizing or inert atmosphere may be used. A nitrogen atmosphere is preferred as being the most economical. The jet nozzles 28 and 29 can serve as the source of the atmosphere within enclosure 27, although additional atmosphere inlets such as the inert 34 may be provided, if required.

A portion of the nitrogen atmosphere within enclosure 27 may be withdrawn and recirculated through the jet finishing nozzles 28 and 29. This is diagrammatically illustrated in Figure 2. The enclosure 27 is provided with an outlet 35. The outlet 35 is preferably connected to a high temperature baghouse 35a for collecting zinc oxide particles. From the baghouse 35a, the atmosphere withdrawn from enclosure 27 passes to a heat exchanger 36. The heat exchanger 36 is connected as at 37 tb the input 38 of a blower 39. The purpose of the heat exchanger is to cool the nitrogen from enclosure 27 ahead of blower 39 to prevent overheating of bearings and seals in the blower. The baghouse 35a could be located between heat exchanger 36 and blower 39, although it is preferred that it be ahead of exchanger 36 to prevent clogging of the heat exchanger fins with zinc dust, the output 40 of blower 39 is connected by conduits 41 and 42 to jet finishing nozzles 28 and 29. The conduits or lines 41 and 42 may contain valves 43 and 44, respectively, so that the plenum pressure of the jet finishing nozzles 28 and 29 can be adjusted. It has been found that through the use of such a baghouse-heat exchanger-blower-sealed conduit system, more than 50% of the high purity nitrogen requirement can be recirculated from enclosure 27 through jet finishing nozzles 28 and 29, thus reducing the nitrogen consumption.

Make-up nitrogen- may be introduced into the system via conduit 45 connected to the line 37 between heat exchanger 36 and the intake 38 of blower 39. The atmosphere recirculation rate is so adjusted as to avoid infiltration of air through the slot 46 through which the coated strip 9a exits enclosure 27.

The jet nozzles 28 and 29 are located to either side of the two side coated base metal strip 9a and directly opposite each other, as shown in Figure 1. However, since the above noted edge problems including wrap-around have been eliminated by the present method,-it is preferred that the jet nozzles 28 and 29 be staggered vertically with respect to each other as shown in Figure 2. This prevents clogging of the nozzles due to zinc splatter and blow-off from one nozzle to the other, as explained above. Either jet knife can be located above the other. The higher of the two jet finishing nozzles (in this instance jet finishing nozzle 28) can be located up to about 0.6 m or more above the bath. The jet finishing nozzles 28 and 29 may be vertically offset with respect to each other by any amount desired.

Generally, they are offset from 5 to 1 5.25 centimeters. Usually the nozzles will be within about 3.8 centimeters of the strip. When the. jet finishing nozzles are offset, the noise level of the finishing step caused by the nozzles is greatly reduced. The jet nozzles 28 and 29 can be of simple construction, having a simple rectangular jet opening and being free of curved lips, shutters, vanes, or other devices. Excellent results have been achieved using jet finishing nozzles having a simple rectangular opening with a uniform width of from 1.25 to 2.05 mm throughout its length.

The enclosure 27 is provided with an exit opening or slot 46 for the two-side coated ferrous base metal strip 9a. Care must be taken to assure that ambient air is not aspirated through the slot 46 due to high gas velocities and turbulent effects operating within the enclosure near the slot 46.

Ambient air aspirated through the slot 46 would cause excessive oxygen to be present in enclosure 27. The use of baffles or additional nitrogen purging around the strip exit 46 may assist in preventing such air aspiration; However, excellent results have been acheived by simply providing a short chimney 47 and locating exit slot 46 atop chimney 47.

The enclosed finishing method of the present invention permits a short immersion, shallow coating pot practice utilizing a partially submerged coating pot roll. This is true because the method of the present invention minimizes the formation of oxide on the surface of the bath and on the partially submerged pot roll. Such a practice has a number of advantages. First of all, it utilizes a smaller coating bath. Furthermore, the amount of the ferrous base metal strip immersed and the duration of immersion are greatly reduced, thereby reducing the amount of iron values going into solution in the coating metal from the ferrous base metal strip.

Figure 3 illustrates such a short immersion, shallow coating pot practice. in Figure 3 a coating pot is shown at 48. The coating pot 48 is similar to coating pot 8 of Figure 2 with the exception that it is shallower. The coating pot 48 contains a bath of molten coating metal 49 which is considerably less volume than the bath 7 of Figure 2.

A snout 50, equivalent to snout 6 of Figure 2, is shown with its lowermost end located beneath the surface of bath 49 so as to be sealed thereby.

The snout 50 contains a turn-down roll 51 equivalent to turn-down roll 25 of Figure 2. A pot roll is illustrated at 52. The pot roll 52 differs from pot roll 26 of Figure 2 in that it is only partially submerged in the molten coating metal bath 49.

The apparatus of Figure 3 includes an enclosure 53 which is equivalent in every way to enclosure 27 of Figure 2 with the exception that its lower edge 53a is bent slightly downwardly and inwardly so as to make a seal with the molten coating bath 49 while at the same time providing clearance for the flight of the uncoated ferrous base metal strip 54 between turn-down roll 51 and the pot roll 52. The coated ferrous base metal strip 54a is shown passing between a pair of jet finishing nozzles 55 and 56 and upwardly through a chimney 57 and an exit slot 58 equivalent to chimney 47 and exit slot 46 of Figure 2.

The operation of the coating and finishing apparatus shown in Figure 3 is substantially identical to that described with respect to Figure 2. Again, jet finishing nozzles 55 and 56 may be connected to a recirculating system (not shown) of the type illustrated in Figure 2. The primary difference between the operation illustrated in Figure 3 and that illustrated in Figure 2 lies in the fact that pot roll 52 is only partially submerged which provides the above noted advantages.

The amount by which pot roll 52 is submerged in bath 49 can be varied. In Figure 3 pot roll 52 is shown more than half submerged. With appropriate configuration of snout 50 and the portion 53a of enclosure 53, the pot roll 52 could be less than half submerged, particularly in those instances where it is desirable to maintain the roll bearings (not shown) above the bath surface.

The pool of molten metal 59 between pot roll 52 and the ferrous base metal strip 54 engaging the pot roll must be of sufficient size to assure adequate coating of the back side or roll side of the ferrous base metal strip. It will be understood that the size of the pool 59 will decrease as the amount by which pot roll 52 is submerged decreases. It is within the scope of the present invention to augment this situation through the use of a grooved pot roll 52 or means to pump additional molten coating metal into the pool 59 (as will be described hereinafter).

Figure 4 illustrates another arrangement to assure adequate coating of the backside or roll side of the ferrous base metal strip in shallow pot practice. In Figure 4 an enclosure is illustrated at 60 which may be identical to the enclosure 27 of Figure 2, having a pair of jet finishing nozzles 61 and 62, an exit chimney 63 and an inlet 64 for the inert or non-oxidizing atmosphere, it will be understood that the enclosure 60 may be provided with the atmosphere recirculation system described with respect to Figure 2. The lower end of enclosure 60 is submerged in a bath 65 of molten coating metal located in a shallow pot 66. Figure 4 also illustrates a conventional snout 67, similar to snout 6 of Figure 2. Again it will be noted that the lowermost end of snout 67 extends below the surface of the molten coating metal bath 65.

The ferrous base metal strip to be coated is shown at 68. The strip passes about turn-down roll 69 in snout 67 and enters the bath as it pass about a first pot roll 70. For pot roll 70 it extends to a second pot roll 71 which directs the coated strip 68a upwardly through enclosure 60.

The amount by which pot rolls 70 and 71 extend into the molten coating bath 65 can be varied. For purposes of this exemplary showing, pot roll 70 and 71 are illustrated as extending into the molten coating metal bath 65 by an amount less than 1/2 their diameters. The existance of the submerged strip 68b between pot rolls 70 and 71 assures adequate coating of the backside or roll side of the ferrous base metal strip. It has been determined that the shallow pot practice of the type just described with respect to Figures 3 and 4 does not significantly change the enclosed nitrogen finishing characteristics or advantages described with respect to Figures 1 and 2.

As has already been made evident, the apparatus of the present invention may utilize various pot roll arrangements. Another arrangement is illustrated in Figure 5 in this Figure, a conventional coating pot is shown at 72 containing a molten coating metal bath 73. A snout 74, equivalent to snout 6 of Figure 2 has its lower end submerged in the molten coating metal bath 73 and is provided with a turn-down roll 75, equivalent to turn down roll 25 of Figure 2. An enclosure 76 has its lower end submerged in the molten coating metal bath 73. The enclosure 76 may be identical to enclosure 27 of Figure 2, having an exit chimney 77 and an atmosphere inlet 78, if required. The enclosure contains a pair of jet finishing nozzles 79 and 80 equivalent to jet finishing nozzles 28 and 29 of Figure 2.Again, the enclosure 76 may be provided with the atmosphere recirculating system (not shown) of Figure 2.

In this embodiment, the ferrous base metal strip 81 to be coated enters the molten coating metal bath 73 and passes about a series of three pot rolls 82, 83 and 84. Rolls 83 and 84 are stabilizer rolls and provide strip shape control, assuring flatness of the coated strip 81 a as it passes between jet finishing nozzles 79 and 80.

In all of the embodiments thus far described, the enclosure and the snout have been illustrated as separate structures. It is also within the scope of the present invention, however, to provide a snout and an enclosure which constitute an integral, one piece structure. This is illustrated in Figure 6.

In Figure 6 a conventional coating pot 85 is shown containing a bath 86 of molten coating metal. The snout-enclosure structure is generally indicated at 87, having a snout portion 87a and an enclosure portion 87b. The snout portion 87a is similar to snout 6 of Figure 2 and has a turndown roll 88 located therein. Turn-down roll 88 is equivalent to turn-down roll 25 of Figure 2. The enclosure portion 87b is similar to enclosure 27 and has an exit chimney 89. The enclosure portion 87b may be provided with an atmosphere inlet 90 equivalent to inlet 34 of Figure 2. Jet knives 91 and 92 are located within the enclosure portion 87b and are in every way equivalent to jet knives 28 and 29 of Figure 2. It will further be understood that the enclosure portion 87b may be provided with an atmosphere recirculating system (not shown) equivalent to that described with respect to Figure 2.In the embodiment of Figure 6, a submerged pot roll is shown at 93.

Under normal circumstances, the snout portion 87a and the enclosure 87b will contain different atmospheres and therefore some sort of seal m-eans should be provided therebetween. The seal means may take any appropriate form. For purposes of an exemplary showing, the seal means is illustrated as being made up of two pairs of sealing rolls 94-95 and 96-97.

It is within the scope of the invention to provide an inlet 98 for an appropriate nonoxidizing gas between sealing rolls 94-95 and sealing rolls 96-97. It is preferable that the nonoxidizing atmosphere between sealing rolls 9495 and sealing rolls 96-97 be at a pressure slightly higher than the pressure of the -atmosphere in snout portion 87a and enclosure portion 87b. This assures that either the enclosure portion 87b or the strip preparation furnace associated with snout 87a can be shut down without contaminating the other. It will also prevent contamination of the atmosphere within hood portion 87b from sources at the entry end of the conventional strip preparation apparatus.

The strip 99 to be coated passes about turndown roll 88 and between sealing roll pairs 94- 95 and 96-97. The strip 99 enters the bath and passes about pot roll 93. Thereafter, the coated strip 99a passes upwardly between jet finishing nozzles 91 and 92, exiting through exit chimney 89. Thus, the operation of the apparatus and the advantages achieved thereby are essentially the same as has been described with respect to Figures 1 and 2.

The unitary snout-enclosure of Figure 6 can also be applied to shallow pot practice. This is illustrated in Figure 7. In Figure 7, the snoutenclosure apparatus is identical to that of Figure 6 and like parts have been given like index numerals. In the embodiment of Figure 7, a shallow pot is shown at 100, containing a shallow bath 102 of molten coating metal. In this instance, a pot roll 103 is shown being partially submerged in the molten metal coating bath 102.

For purposes of this exemplary showing, the pot roll 103 is illustrated as being submerged by an amount less than half its diameter. The pot roll 103 could, of course, be submerged by an amount more than half its diameter, as is shown with respect to pot roll 52 of Figure 3. It would even be possible to provide the apparatus of Figure 7 with a pair of pot rolls of the type described with respect to Figure 4.

In Figure 7, however, for purposes of an exemplary showing the apparatus is illustrated as being provided with a pump for the molten coating metal of bath 102, the outlet of the pump being shown at 104. The pump outlet 104 creates a pool 105 of molten coating metal between the ferrous base metal strip 99 and pot roll 103, which pool assures adequate coating of the back or roll side of the ferrous base metal strip. Such a pump for the molten coating metal could be provided for the embodiment of Figure 3, if the pool 59 of Figure 3 were inadequate. In all embodiments of the present invention, where the pot roll is only partially submerged,.it would be within the scope of the invention to use a grooved pot roll. The grooves carry molten coating metal to the roll side of the ferrous base metal strip.

Since the method of the present invention eliminates the oxide problems with respect to the strip and strip edges, it has been found that the relatively heavy coatings can be achieved at lower line speeds, such coatings having excellent surface characteristics. For example, with minimum controlled wipe by the jetfinishing nozzles, coating weights of up to about 543 g/m2 (about 1.78 ounces per square foot) having been achieved at a line speed of 1 2 meters per minute in the laboratory.

A laboratory galvanizing line utilizing a 10.16 cm strip was provided with an enclosure similar to enclosure 27 of Figure 2. The chimney 47 was 15.25 cm high and provided with an exit slot 46 having a width of 31.75 mm and a length of 1-2.7 cm. The enclosure was provided with a pairs of jet finishing nozzles (equivalent to jet nozzles 28 and 29 of Figure 2), each having a slot-like opening of 1.27 mm width throughout its length. The lower of the two jet finishing nozzles was maintained at a distance of four inches from the bath surface.

The other jet nozzle was-offset vertically and upwardly therefrom by 1 2.7 mm. The jet nozzles were maintained at a distance from the strip of about 6.35 mm. The enclosure was provided with a recirculating system of the type shown in Figure 2. Make up nitrogen was added at the rate of 85 m3 per hour and nitrogen atmosphere within the enclosure was maintained at a pressure of 12.7 mm of water.

The ferrous base metal strip was 0;381 mm cold rolled steel witch relatively smooth surfaces at 1.27 ssm and 3.546 Kp/m. During this run, a coating of 183 g/m2 was being produced and a line speed of 21.4 m per minute was used. The influence of oxygen contamination in the enclosure containing high purity nitrogen was evaluated by metering compressed air into the recirculating system in increasing amounts until defects were observed in the molten coating.

With oxygen below tO ppm the molten coating was-glossy, smooth, free of visible oxide and without sign of edge problems. The solidified coating showed a dead flat spangle without spangle boundary relief. As the oxygen was purposely increased, no ripples occurred at an oxygen level of 140 ppm. Detrimental finishing effects were first observed at an oxygen level within the enclosure of about 200 ppm in the form of edge oxide berries, ripples, a ridge of heavy edge metal, and some spangle relief. These conditions become steadily more pronounced as the oxygen level was increased to 600 ppm.

Surface oxide bands developed when the oxygen level reached about 700 ppm. These oxide bends extended inwardly from the strip edge and increased to gross feathers of oxide when the oxygen level reached 850 ppm.

This run showed that the enclosed nitrogen finishing method of the present invention produces a smooth, uniform hot-dip zinc coating finish without the common ripple, dross, oxide curtain and edge build-up defects associated with conventional edge finishing. A dead flat spangle can be produced suitable for temper rolling for extra smooth applications and eliminating the ne-ed for minimizing spangle practices. Simplified jet finishing nozzles with uniform slot openings can be used and can be vertically offset without zinc splatter and without heavy edge coating or coating wrap-around. The noise level of the finishing step was drastically reduced not only by virtue of the fact that the nozzles can be offset with respect to each other. The relationship between oxygen contamination of the finishing gas and the coating surface quality was clearly demonstrated. The oxygen level within the enclosure should be maintained at less than about 200 ppm and preferably less than about 100 ppm. In other similar runs nitrogen at the rate of 85 m3 per hour was circulated through the jet finishing nozzles using about 42.5 m3 per hour or less make-up nitrogen, confirming the ability to recirculate more than 50% of the high purity finishing gas requirement.

In all of the embodiments illustrated in the Figures, the enclosure-is shown in semidiagrammatic fashion. It will be understood- by one skilled in the art that the enclosure will be provided with suitable support means and the like. Furthermore, the enclosure may be removable in whole or in part for maintenance or if regular air finishing is to be practiced.

Modifications may be made in the invention without departing from the spirit of it.

Claims (28)

Claims

1. A finishing process for continuous hot-dip, two-sided coating of a ferrous base metal strip with a molten coating metal of the type wherein said ferrous base metal strip is caused to enter a bath of said molten coating metal contained in a coating pot, said ferrous base metal strip having been treated to bring it to a coating temperature sufficiently high to prevent casting of said coating metal thereon and low enough to prevent excess coating metal-base metal alloying and to render the surfaces of said strip clean and free of oxide as it passes through said molten coating metal bath, including the steps of providing an enclosure in sealed relationship with said bath for said two-side coated ferrous base metal strip as it exits said bath and an exit in said enclosure for said coated strip, maintaining an non-oxidizing atmosphere within said enclosure, locating a jet finishing nozzle to either side of said coated strip within the said enclosure, jet finishing said coated strip with a non-oxidizing gas and maintaining said jet finishing gas and said atmosphere within said enclosure at an oxygen level of less than about 200 ppm, whereby to render said two-side coated strip free of oxide' -caused coating nonuniformities.

2. The process claimed in claim 1 wherein said molten coating metal is chosen for the class consisting of zinc, zinc alloys, aluminum, aluminum alloys, terne and lead.

3. The process claimed in claim 1 or claim 2 including the step of maintaining said jet finishing gas and said atmosphere within said enclosure at an oxygen level of less than about 100 ppm.

4. The process claimed in any of claims 1 to 3 wherein said non-oxidizing jet finishing gas and said atmosphere within said enclosure comprise nitrogen.

5. The process claimed in any one of claims 1 to 3 wherein said non-oxidizing jet finishing gas and said atmosphere within said enclosure comprise an inert gas.

6. The process claimed in any preceding claim including the step of vertically staggering said jet finishing nozzles with respect to each other.

7. The process claimed in any preceding claim wherein each of said jet finishing nozzles has a rectangular nozzle opening of uniform width throughout its length and equidistant from said coated strip throughout its length.

8. The process claimed in claim 4 including the step of recirculating at least 50% of said nitrogen from said enclosure through said jet finishing nozzles.

9. The process claimed in claim 5 including the step of recirculating at least 50% of said inert gas from said enclosure through said jet finishing nozzles.

10. The process in claim 8 including the step of vertically staggering said jet finishing nozzles with respect to each other.

11. The process claimed in any preceding claim wherein each of said jet finishing nozzles has a rectangular nozzle opening of uniform width throughout its length and equidistant from said coated strip throughout its length.

12. Finishing apparatus for use with a conventional coating line for the hot-dip, two-side coating of a ferrous base metal strip with a molten coating metal, said coating line being of the type having a coating pot, a bath of molten coating metal within said coating pot, means to conduct said ferrous base metal strip through said molten coating metal bath, strip preparation means to bring said ferrous base metal strip to a coating temperature sufficiently high to prevent casting of said coating metal thereon and low enough to prevent excess coating metal-base metal alloying and to render the surfaces of said strip clean and free of oxide as it passes through said molten coating metal bath, comprising an enclosure for said strip as it exits said molten coating metal bath, said enclosure having an open bottom end extending into said molten coating metal bath, said enclosure having an exit slot formed therein for said strip, a pair of jet finishing nozzles located within said enclosure, above said molten coating metal bath and to each side of said strip, means to maintain a non-oxidizing gas within said enclosure having an oxygen level of less than about 200 ppm, and means to supply said jet finishing nozzles with a non-oxidizing gas having an oxygen level of less than about 200 ppm.

13. The apparatus claimed in claim 12 wherein said molten coating metal is chosen from the class consisting of zinc, zinc alloys, aluminum, aluminum alloys, terne and lead.

14. The apparatus claimed in claim 12 or claim 1 3 wherein said jet finishing nozzles are staggered vertically with respect to each other.

15. The apparatus claimed in any of claims 12 to 14 wherein each of said jet finishing nozzles has a rectangular nozzle opening of uniform width throughout its length and equidistant from said coated strip throughout its length.

1 6. The apparatus claimed in any preceding claim including means to recirculate at least 50% of said non-oxidizing gas within said enclosure from said enclosure through said jet finishing nozzles.

17. The apparatus claimed in any one of claims 12 to 1 6 wherein said strip preparation means is of the type having a snout extending therefrom into said molten coating metal bath and means within said snout to conduct said ferrous base metal strip from said strip preparation means into said molten coating metal bath, said enclosure comprising an integral part of said snout and sealing means within said snout to isolate said non-oxidizing gas within said enclosure from said strip preparation means.

18. The apparatus claimed in any of claims 12 to 1 7 including at least one pot roll about which said strip passes through said bath and is directed upwardly out of said bath and into said enclosure.

19. The apparatus claimed in any of claims 12 to 19 including a short chimney on said enclosure, said chimney having a lower end communicating with the interior of said enclosure, said chimney having an upper end provided with said exit slot for said strip.

20. The apparatus claimed in claim 1 6 wherein said recirculating means comprises an outlet for said non-oxidizing gas within said enclosure, a bag house connected to said outlet, a heat exchanger connected to said bag house, a blower connected to said heat exchanger, said blower having an outlet connected to said jet finishing nozzles and means within said recirculating system to add m-ake-up non-oxidizing gas.

21. The apparatus claimed in claim 18 wherein said at least one pot roll is fully submerged within said molten coating metal bath.

22. The apparatus claimed in claim 18 wherein said at least one pot roll is submerged within said molten coating metal bath by a distance greater than half its diameter.

23. The apparatus claimed in claim 18 wherein said at least one pot roll is partially submerged within said molten coating metal bath by a distance less than half its diameter.

24. The apparatus claimed in any of claims 12 to 23 including a pair of stabilizer rolls within said bath and so positioned therein as to cause said strip to follow a tortuous path thereabout after passing about said at least one pot roll and before exiting said molten coating metal bath to assure flatness of said strip as it passes between said jet finishing nozzles.

25. The apparatus claimed in any of claims 18, 21,22 or 23 wherein said at least one pot roll comprises a grooved pot roll.

26. The apparatus claimed in any one of the preceding claims including a pump for molten metal from said molten metal bath, said pump having an outlet so positioned as to form a pool of said molten metal between said strip and said at least one pot roll on that side of said pot roll where said strip first contacts said pot roll to assure adequate application of said molten coating metal on the roll side of said strip.

27. A finishing process for ferrous base metal strip substantially as hereinbefore particularly described and as illustrated in the accompanying drawings.

28. A finishing apparatus for ferrous base metal strip substantially as hereinbefore particularly described and as illustrated in the accompanying. drawings.