US3379557A

US3379557A - Suppression of visible spangle

Info

Publication number: US3379557A
Application number: US380533A
Authority: US
Inventors: George R Hoover; Paul E Schnedler
Original assignee: Armco Inc
Current assignee: Armco Inc
Priority date: 1964-07-06
Filing date: 1964-07-06
Publication date: 1968-04-23
Anticipated expiration: 1985-04-23
Also published as: DE1521109C3; GB1117323A; AT268817B; DE1521109A1; DE1521109B2

Description

April 23, 1968 G. R. HOOVER ET AL 3,379,557

SUPPRESSION OF VISIBLE SPANGLE Filed July 6, 1964 INVENTOR Geezer: R. HOOVER AND PAUL E. SCHNEDLER,

ATTORNEYS.

United States Patent Q 3,379,557 SUPPRESSION F VlSlBLE SPANGLE George R. Hoover and Paul E. Schnedler, Middletown,

()hio, assignors to Armco Steel Corporation, Middletown, Ohio, a corporation of Ohio Filed July 6, 1964, Ser. No. 330,533 12 Claims. (Cl. 117-64) ABSTRACT OF THE DISLOSURE A method of suppressing visible spangle in non-alloy coatings on metallic strips by inducing substantially submacroscopic spangling. A water solution of an inorganic salt is applied to a freshly coated strip at a point where the temperature of the molten coating metal on the strip is above its solidification temperature. The inorganic salt is selected from the class consisting of inorganic salts which decompose in the range of 175 to 550 F. and those salts which will hydrolize when added to water to form inorganic salts capable of decomposing in the above stated temperature range.

This invention relates to spangle metallic coatings wherein the amount of visible spangle is suppressed. Spangle is the name given to the appearance of metallic coating on metal strip which is characterized by visible crystalline boundary outlines.

Where metal coatings, as for example zinc coatings, are to be exposed to view without painting or other after treatment, spangle has generally been considered a desirable characteristic because it gives an attractive appearance to the finished product.

Where a metal product having a metallic coating thereon is to be painted, however, the spangle of the coating is considered undesirable because the crystalline boundaries, to a certain extent, show in the painted finished product.

It is, of course, possible to provide for full alloying of the coating with the base metal and when such alloying takes place, no spangle is apparent but heretofore there has been no truly satisfactory way of producing nonalloy coatings without some visible spangle.

Accordingly, it is an object of the present invention to provide a method of treating freshly coated metallic strip to minimize the amount of visible spangle. More particularly, it is an object of the present invention to provide a method for inducing a multitude of closely spaced, relatively minute, spangles at a precise point in the cooling cycle of the coating so that the spangle which results in a sub-macroscopic spangle, or nearly so, i.e., a spangle which if visible at all to the naked eye, is just barely so visible. A surface having such a sub-macroscopic spangle can be painted without the problem of the crystalline boun ary showing through.

It is also an object of the present invention to provide means by which this method may be carried out. In carrying out the method, it is important that solidification nuclei be applied to the coating as it is about to freeze. Since the coating on a strip emerging from a coating bath does not exhibit constant temperature graoient varying with distance traveled from the bath, it is another object of the invention to provide means for insuring that the solidification nuclei will always be applied to the coating when the coating is at a predetermined temperature.

A very important object of the invention is to provide solidification nuclei which greatly enhance the formation of the multitude of closely spaced, relatively minute, spangles which will give, to the naked eye, a coated strip with a substantially unspangled appearance.

These and other objects of the invention which will be pointed out in more detail hereinafter, or which will be apparent to one skilled in the art upon reading these specifications, are accomplished by that method which includes the use of the herein disclosed solidification nuclei, and of which method the following description discloses an exemplary embodiment.

Reference is made to the single drawing representing a diagrammatic showing of the invention.

Briefly, in the practice of the invention, a metallic strip is first coated in any desired manner. According to the present invention, a multitude of solidification nuclei are then applied to the molten coating when such coating is at a temperature very close to the solidification point (or freezing point) of the coating metal. These nuclei are applied in a band transversely of the direction of strip travel, the band having such a width, having regard to the particular solidification nuclei being employed, that the coating metal is molten as it enters the band and solid as it leaves the band.

The solidification nuclei of this invention consist of that group of reactive inorganic salts which decompose in the range of to 550 F. Of this group di-basic ammonium phosphate (which decomposes at 180 to 210 F.) is preferred. It has been determined that the salts of this group, applied to the galvanized surface as taught herein, produce a highly satisfactory surface having an unspangled appearance. The use of these salts does not foul up the equipment and Work area; these salts and their decomposition products, do not promote corrosion or deterioration of equipment or buildings. They are safe to use-- they, their decomposition products, are non-toxic and will not harm the operator nor cause him unpleasantness. These salts are largely readily available and economical to use. No harmful residue is left permanently on the finished product. Indeed it is believed that some of these salts even enhance the product, over and beyond produc ing the unspangled appearance, by leaving on the surface substances which promote paintability, e.g., the phosphate which remains when di-basic ammonium phosphate is employed as the solidification nuclei. In some other instances wherein the use of substances from the designated group leaves an undesirable residue, such as occurs, for example, when sodium bicarbonate is used, such residue, in this case being sodium hydroxide (N OI-I), can be removed by a simple water rinse.

Other representative materials which fall within the designate group are sodium bicarbonate (which decomposes at 498 F.), sodium phosphate, other phosphates, ammonium borate, ammonium molybdate and ammonium sulfate. These are all reactive salts. All decompose or disassociate at temperatures below the temperature of molten zinc and release radicals known to be reactive with metallic zinc. Ammonum sulfate decomposes at 536 F. and ammonium molybdate at 270 to 300 P. All of the reactive salts, named herein by way of examples of the discovered class of solidification nuclei, have decomposition temperatures in the range of 175 to 550 F.

Salts that have decomposition temperatures much below 175 F. appear to be too fugitive to produce the desired results. Ammonium bicarbonate, for example, decomposes at 104 F. and has been found to be of little value. Salts having decomposition temperatures above 550 F. have little or no effect; their solutions are rated no better than Water.

There are some materials which may initially appear to be outside the designated group but which, in use, will fall within the group, which may be explained as follows. Sodium carbonate, for example, which is stable as such at temperatures above the melting point of zinc (787 F.), is a salt which is included with the designated group because when it is dissolved in water it reacts to form sodium bicarbonate (decomposition temperature=498 F.) and sodium hydroxide:

Na- CO NaHCO Another example of this type is tri-sodium phosphate (not in the range as such) which reacts with water to form sodium phosphate (which is in the range) and sodium hydroxide. Substances having the hydroxyl radical may, as indicated, form a non-desirable residue on the strip, but which residue is easily rinsed otf.

Thus the designated solidification nuclei of this invention, described as reactive inorganic salts which decompose in the range of 175 to 550 F., must be considered as including those salts which, although initially outside the designated range when considered alone, will hydrolize when added to water to form a salt within the range plus an hydroxide which can be rinsed off.

Referring now to the figure, which. is an example of one way by which a strip may be galvanized, a coating pot is shown at containing a body of molten coating metal 11. The pot is provided with the usual pot roll 12 and a strip of metal 13, which has been given any desired pretreatment, is led into the pot and around the roll 12 and exits vertically upward through an exit machine 14. The coated strip 13a passes vertically upward to a turning roll 15 and thence to suitable coiling apparatus or the like. The por tion of the apparatus heretofore described is in all respects conventional and will not be described in further detail. It is to be understood that the invention is applicable to galvanizing processes in general and is not restricted to any particular mode of applying the zinc coating.

From the method standpoint, the minimizing of spangle by inducing sub-macroscopic spangling is accomplished by applying a multitude of the designated solidification nuclei to the molten coating of the strip. The process comprises spraying the strip or sheet while the zinc coating is still in the molten state as will be further described below, with a solution of the selected reactive inorganic salt in water. The spray may be in the form of a moderately fine mist aspirated by means of air or steam, or it may be produced by pressure of the solution through properly designed nozzles. Preferably standard paint spray guns may be employed to apply the treating solution. An example of this type gun is a Binlcs Model 21 automatic spray gun equipped with a #66 nozzle and a .070 inch orifice fitted with a 66PE air cap. Other guns with comparable characteristics may be used with equally satisfactory results. In the preferred application of the invention di-basic ammonium phosphate is used as the reactive inorganic salt and steam as the aspirating media.

The point in the travel of the coated strip at which the solidification nuclei are applied is quite critical. If the temperature of the coating is too high when the nuclei are projected onto the coating, the action of inducing submacroscopic spangling is ineffective. If the temperature is too low, then of course solidification with normal spangle formation has already occurred. Generally speakin g, therefore, the particles must contact the surface of the molten coating while the temperature of that surface is just above the freezing point of the coating metal. Zinc, for example, is considered as having a freezing point of 787 F. and the particles should contact this coating metal at a temperature just above the freezing point, preferably up to about 10 F, or even up to 25 to 40 F., thereabove, depending on the quenching qualities of the cooling media.

The coating temperature at which the spray of solidification nuclei must contact it will depend also upon the cooling or quenching qualities of the spray. If a spray of a high volume of water is used, there will be a greater quenching action and therefore the coating could be hotter at the point where it is sprayed. If a fine spray of these particles is used, the temperature of the coating should be only very slightly above the solidification temperature. If an aqueous solution is used, the volume of the solvent must be considered with respect to its quenching action and it must be insured that the solvent does not quench the coating to a temperature below its freezing point before the solute particles can serve their function as solidi fication nuclei. Furthermore, the use of large quantities of water is to be avoided because there is the problem of disposing of that portion which is not converted to steam and because such a volume will disrupt the smoothness of the surface.

From what has gone before, it will be understood that one of the most important requirements in the effective practice of this invention is the application of the solidification nuclei at a rather precisely defined coating temperature. This cannot be accomplished by the positioning of the spray heads at a fixed distance above the bath. Variations in bath temperature, variations in' speed of movement of the strip, variations in strip temperature, and even currents in the ambient air in the plant, which variations are known to occur from time to time in commercial coating processes, will produce variations in the cooling rate of metallic coatings on the strip. Thus, the freezing or spangle forming temperature (theoretically 787 F. in the case of pure zinc) will almost never occur at a constant distance from the bath. The point at which that particular freezing temperature obtains will move up and down during the galvanizing and spangle suppression process, and as it moves up and down, the spray heads or other projecting apparatus should also move up and down to compensate for this. Also, it should be understood that pure zinc is not the only galvanizing coating that may be used; the galvanizing spelter commonly includes additives and even impurities which will change the freezing point, and there fore the spangle forming point,.of.the bath material. In any event, however, the point at which the application of the solidification nuclei is most effective is in the previously designated range of say up to 10 F., or even up to 25 to 40 F., above the freezing point of the particular bath employed.

Referring again to the figure, there isdiagrammatically indicated a vertical guide 16 disposed adjacent the vertically moving coated strip 13a. An element 17 is movable vertically along the guide 16 and carries the spray heads 18 and a sensing device 19. The sensing device 19 is a radiation pyrometer or infrared pyrometer. Such devices are available commercially and have the advantage that they can measure temperature in the ranges here under consideration quite accurately without contacting the strip.

A signal from the radiation pyrometer 19 actuates a.

conventional electronic control device .20 which is also commercially available, and the electronic control device is operatively connected to a reversible electrically powered hoist motor 21. The hoist motor cable 22 is connected to the element 17 so that when the motor 21 operates in one direction, it raises the element 17 together with the pyrometer 19 and spray heads wand when the motor 21 runs in the opposite direction, it lowers the element 17. By this apparatus, which in its individual elements is commercially available and not a part of the present invention, the control device 20 and the hoist motor 21 will maintain the spray heads 18 at the desired point.

It will be understood that the sprays from the spray head 18 extend entirely transversely across the strip and that the width of the spray in the direction of strip travel will be such, having due regard to the material being sprayed, that the coating which enters the band in a molten condition will leave the band in a solid condition.

It will be seen, therefore, that there has been provided a fully automatic adjustment of the point at which the sprayed material is applied to the coating and that the sprayed material will therefore be applied to the coating at the optimum point in spite ofrchanges. in various operating conditions or ambient temperature and draft conditions.

In closing it is pointed out that the most relevant prior art known to the inventors are the United States patents Cook et al. 2,126,244 and Coifman 2,764,808. Cook et al. disclose the spraying of galvanized ware with a water mist or a water solution of copper sulphate, sodium nitrate, sodium chloride, potassium chromate or potassium permanaganate in an efiort to produce Ware having an unspangled appearance. Cotfman discloses the concept of automatic mechanism controlled by the temperature of the coating bath for automatically adjusting the vertical position of bonding rolls in a system wherein layers of fibrous material are secured to an iron or steel sheet.

It will be understood that modifications may be made without departing from the spirit of the invention and no limitations are intended other than as specifically set forth in the claims which follow.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In the hot dip coating of a ferrous metal strip with molten zinc coating metal, the method of suppressing visible spangle by inducing substantially submacroscopic spangling, including the step of applying to the freshly coated strip, at a point where the temperature of said molten coating metal on said strip is above its solidification temperature, a water solution of a salt selected from the class consisting of reactive inorganic salts which decompose in the range of 175 to 550 F. and release radicals reactive with metallic zinc and those salts which will hydrolize when added to Water to form a reactive inorganic salt decomposable in said range, said salts and their decomposition products being non-toxic and leaving no harmful, difiiculty removable residues on said coated strip.

2. The method of claim 1 in which said solution is applied at a point where the temperature of said molten coating metal on said strip is up to F. above its solidification temperature.

3. The method of claim 1 in which said solution is applied at a point where the temperature of said molten coating metal on said strip is up to 40 F. above its solidification temperature.

4. The method claimed in claim 1 including the step of causing the temperature of said molten coating metal on said strip to automatically control the distance from said bath at which said aqueous solution is applied to said strip.

5. The method claimed in claim 1 wherein said salt is chosen from the class consisting of di-basic ammonium phosphate, sodium bicarbonate, sodium phosphate, ammonium borate, ammonium molybdate, ammonium sulfate, sodium carbonate and tri-sodium phosphate.

6. The method according to claim 1, wherein a spray of said aqueous solution is applied to the strip throughout a band transverse to the length of the strip, said band being disposed at such a distance from the bath that the coating is still molten as it enters said band, and is solid as it emerges from said band.

7. The method of claim 1 including the step of subjecting the strip to a water rinse after the substantially submacroscopic spangling has been induced.

8. The method of claim 1 wherein said reactive inorganic salt is di-basic ammonium phosphate.

9. The method of claim 1 wherein said reactive inorganic salt is sodium bicarbonate.

10. The method of claim 1 wherein said reactive inganic salt is sodium phosphate.

11. The method of claim 1 wherein said reactive inorganic salt is sodium carbonate.

12. The method of claim 1 wherein said reactive inorganic salt is tri-sodium phosphate.

References Cited UNITED STATES PATENTS 2,126,244 '8/1938 Cook et a1. 117131 X 2,764,808 10/1956 Coifman 29473.1 2,950,991 8/1960 Seymour 1171l4 X RALPH S. KENDALL, Primary Examiner.