The method and the apparatus of the present invention are applicable to a 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 optimum 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 to fluxless, hot-dip metallic coating of ferrous base metal strip where 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. Pat. Nos. 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. Pat. No. 3,320,085.
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 535.degree. C. to about 760.degree. C. 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 from about 650.degree. C. to about 925.degree. C. and thereafter cooled approximately to the molten coated 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. Pat. No. Re 29, 726--U.S. Pat. Nos. 3,837,790--4,123,291--4,123,292 and 4,140,552. The above mentioned prior art patents constitute non-limiting examples of fluxless, 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 U.S. Pat. Nos. 2,824,020 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 the 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 surface 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. Some of these drawbacks include coating ripples, oxide curtains and, bath surface oxide problems, including the necessity for top skimming. All of these defects are real, but are reasonably controllable by known methods.
U.S. Pat. Nos. 4,107,357 and 4,114,563 and German Pat. No. 2,656,524 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 non-oxidizing gas. However, the primary purpose 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. Nevertheless, U.S. Pat. No. 4,114,563 mentions in passing that the above noted defects are controlled or eliminated when finishing in a protective atmosphere.
A 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 buildup or spooling will occur when the continuous strip is coiled under tension.
Edge berries (small balls of oxide) which are attached to the strip edge and are pulled through the jet blast constitute another troublesome problem. 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 the jet nozzle to its ends. Such a contoured jet finishing nozzle is taught in U.S. Pat. No. 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, both internal 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 minumum of operator attention, maximum coating metal economy, and proper edge control over a wide range of strip widths and line speeds.
Edge build up in a one-side coating process differs from that encountered in a conventional two-side coating process. This is true because, when jet finishing is employed, it is normally performed with a single jet nozzle, preferably with a back-up roll on the uncoated side of the strip. As a result, edge build-up is less severe, since the back-up roll serves as an extension of the strip edges.
Yet another major hot-dip zinc coating defect is commonly referred to as "spangle relief". 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. Spangle relief can be reduced by such methods as purposely causing part of the zinc coating to alloy with the ferrous base metal, 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, U.S. Pat. Nos. 3,322,558; 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. However, the results achieved by these spangle minimizing methods are not always consistent.
While spangle relief can be reduced or masked by temper (skin-pass) rolling such temper rolling causes these defects 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 and appliance parts are stamped or formed. These defects may not be completely masked after the parts are painted.
Coating irregularity problems constituted a major reason why prior art workers have recently turned their attention to a one-side coated product (as exemplified by U.S. Pat. No. 4,082,868) wherein the uncoated side is the side to be painted on critical surface items made therefrom, even at the sacrifice of corrosion resistance on the non-galvanized side.
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 minimizing jet nozzle to strip distance and reducing the jet finishing pressure will minimize the prominence of coating ripples. However, low jet finishing pressure and close positioning of the jet finishing nozzles at the same time create an edge build-up problem. Prior art workers therefore have had 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 about 100 feet per minute (30 meters per minute) to produce commercial class coating weight (ASTM A525, G90). Edge build-up problems on G-90 coating commonly occur at speeds below about 150 feet per minute (45 meters per minute). Heavier coatings are even more difficult to control edge-to-edge uniformly.
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. Vertically offset jet nozzles cause a wrap-around effect with a bead of heavy coating metal along the edge on the side opposite the last nozzle operating on 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 hot-dipped, two-side 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 when nitrogen is used than air when air is used. However, the results achieved by such finishing are more nearly like those achieved in jet finishing with air in an ambient atmosphere than like the results achieved by the present method.
U.S. Pat. Nos. 3,505,042 and 3,505,043 teach hot dip coating of a ferrous metal with magnesium-zinc alloy and magnesium-aluminum-zinc alloy respectively. The first of these references utilized coating rolls for coating control and nitrogen gas solely for cooling after finishing between exit rolls. The second of these references does not disclose specific finishing means, but does enclose the coating apparatus in an atmosphere of 10% hydrogen with the balance nitrogen. The reference also mentions blowing nitrogen gas over the coated strip being withdrawn from the bath to rapidly cool the coating and to control coating weight, the nitrogen gas having a temperature below 100.degree. F. (40.degree. C.) and preferably about 50.degree. F. (10.degree. C.).
The present invention is based upon the discovery that if, in a conventional, continuous, hot-dip, two-side galvanizing process, the coating 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 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 enables the practice of any of the above noted spangle minimizing techniques, greatly enhancing the results achieved and the uniformity of spangle minimization throughout the width of the strip. Furthermore, the markedly improved results are consistent.
The most significant aspect of the present invention, however, is the discovery that all coating control problems at the strip edges are completely eliminated with the substantial 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 G-90 coatings can be produced without difficulty at speeds as low as 30 feet per minute (9 meters per minute).
Coating control at the strip edges is a very real problem in a two-side coating process where jet finishing cannot be accomplished against a back-up roll as in one-side coating processes. In the practice of the teachings of the present invention, heavy coating at the strip edges does not occur. Jet nozzles can be vertically offset, eliminating the need for precise positioning, greatly reducing noise, and eliminating the hazard of zinc splatter plugging opposed nozzles. Jet nozzle design may be simplified to utilize a slot-like nozzle opening of uniform width throughout its length, rendering unnecessary the multitude of special jet nozzle designs, methods and accessories which have been used for controlling edge build-up. Superior uniform coating thickness results edge-to-edge for all coating weights because the center profile need no longer be distorted to compensate for heavy edges. As a result, coils can be wound under high tension without spooling. Furthermore, with uniform coating weight edge-to-edge, less coating metal is used. The greatly reduced noise, by virtue of vertically offsetting the jet nozzles in an enclosure, represents a major step in safety and environment improvement. The simplified jet nozzle design results in more uniform coating, more uniform cooling and a flatter strip.
Heretofore, in the practice of conventional two-side jet finishing, neither the mechanism for 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 U.S. Pat. No. 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 wrap-around to occur when the jet nozzles are vertically offset.
Edge build-up of the molten coating metal is eliminated by the practice of the present invention by avoiding oxidation.
The zinc coated product produced by the method of the present invention (including spangle minimization) has such excellent surface qualities after temper rolling that it is suitable for use in exposed automotive body panels, appliance applications and the like.