US3756844A - Zed spangle produkt controlling cooling of galvanized strip in process of forming minimiz - Google Patents

Abstract

METHOD FOR CONTROLLING THE COOLING PATTERN OF A MOLTEN METALLIC COATING ON A FERROUS STRIP AFTER IT EMERGES FROM THE BATH OF MOLTEN METAL, WHEREIN A HEATING AND/OR A COOLING FLUID IS DIRECTED AT SELECTED LONGITUDINAL ZONES OF THE STRIP. THE APPARATUS INCLUDES A MANIFOLD ARRANGED TRANVERSELY OF THE PATH OF MOVEMENT OF THE STRIP AND EXTENDING ACROSS ITS FUL WIDTH. THE MANIFOLD WILL BE SUPPLIED WITH A HEATING OR COOLING FLUID, AND A PLURALITY OF NOZZLES DIRECTED AT SELECTED LONGITUDINAL ZONES OF THE STRIP. EACH NOZZLE IS INDEPENTENTLY CONTROLLED SO THAT THE FLUID IS DIRECTED ONLY AT THE DESIRED LONITUDINAL ZONES OF THE STRIP.

Description

Sept, 4, 1973 E. C. BUNNELL ETAL CONTROLLING COOLING OF GALVANIZED STRIP IN PROC ESS 0F FORMING MINIMIZED SPANGLE PRODUCT Filed July 30, 1971 PAUL E'- sax/#504512 NVENTOR/S 5044000 6'- EVA/HELL BY ym,

ATTORNEYS United States Patent O 3,756,844 CONTROLLING COOLING OF GALVANIZED STRIP IN PROCESS OF FORMING MINIMIZED SPANGLE PRODUCT e Edmond C. Bunnell, Trenton, and Paul E. Schnedler,

Middletown, Ohio, assignors to Armco Steel Corporation, Middletown, Ohio Filed July 30, 1971, Ser. No. 167,726 Int. Cl. C23c 1/02 US. Cl. 117-64 R 6 Claims ABSTRACT OF THE DISCLOSURE Method for controlling the cooling pattern of a molten metallic coating on a ferrous strip after it emerges from the bath of molten metal, wherein a heating and/or a cooling fluid is directed at selected longitudinal zones of the strip.

The apparatus includes a manifold arranged transversely of the path of movement of the strip and extending across its full width. The manifold will be supplied with a heating or cooling fluid, and a plurality of nozzles directed at selected longitudinal zones of the strip. Each nozzle is independently controlled so that the fluid is directed only at the desired longitudinal zones of the strip.

BACKGROUND OF THE INVENTION This invention relates to a method and apparatus for the selective heating and/or cooling of desired longitudinal areas of a moving strip. The invention has great and particular utility in controlling the cooling pattern of a molten metallic coating applied to a ferrous strip.

Hot dip metallic coating processes, generally considered, include three steps. The first involves a thorough cleaning of the base metal and perhaps some additional treatment to render the surface of the strip receptive to the molten coating metal.

The thoroughly cleaned strip is then passed at a predetermined temperature into a bath of molten coating metal. As the strip is withdrawn from the molten metal bath, it will carry with it a layer of still molten coating metal.

This still molten coating must be solidified before the strip can be coiled or otherwise subjected to further processing.

Many metallic coatings, and particularly zinc coatings, form a visible spangle during the solidification process. Where the metallic coating is not subjected to any further treatment such as paint or the like, the spangle may be considered desirable in that it gives a distinctive appearance to the finished product.

However, in many cases, such as where the metallic coated product is to be painted, the spangle is very undesirable because the crystalline boundaries will show through the finish paint coat.

US. Pat. No. 3,379,557 issued on Apr. 23, 1968, in the names of George R. Hoover and Paul E. Schnedler, describes and claims a method wherein the visible spangle may be suppressed by inducing substantially sub-macroscopic spangling. According to the teachings of this patent, 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 is rather close to its solidification point. Preferably, this water solution will be applied in a band extending across the strip; that is, transverse to its direction of movement.

The Water solution of an inorganic salt provides a multitude of closely spaced solidification nuclei, which induce a multitude of closely spaced, minute spangles which are sub-macroscopic, or nearly so. Practice of the teachings of this patent has successfully produced a zinc coated steel strip having an unspangled appearance.

It was pointed out in this patent that the time at which the solidification nuclei are applied is quite critical. It the temperature of the still molten coating is too high, the action of inducing sub-macroscopic spangling is inefiective. On the other hand, if the temperature is too low, then the solidification and normal spangle formation will have already occurred. To this end, the Hoover et al. patent under discussion teaches that the spray heads which apply the solidification nuclei must be movable vertically, in order to control the point at which the solidification nuclei are applied.

Recent experience in the commercial production of a minimized spangle product has taught that the cooling pattern of the molten zinc coating is by no means uniform. That is, many variables must be taken into account. Specifically, changes in gauge of the base metal, strip width, coating weight, ambient temperature and atmospheric conditions, and the like all affect the cooling pattern of the molten zinc. Other workers in the art have endeavored to produce a more uniform cooling pattern by the provision of edge burners to heat only the edges of the strip, while still other workers have attemped to cause a forced cooling of the entire width of the strip at the same rate.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a schematic view showing the normal cooling pattern of molten metal on a base metal strip.

FIG. 2 is a schematic view similar to FIG. 1 showing the desired cooling pattern.

FIG. 3 is a schematic view showing the apparatus of this invention for selectively heating or cooling predetermined longitudinal areas of the coated strip.

FIG. 4 is a schematic view showing a modification for the invention permitting simultaneous heating and cooling of different, selected longitudinal zones on the strip.

SUMMARY OF THE INVENTION Broadly considered, this invention relates to a method and apparatus for controlling the cooling and hence solidification of a molten metal coating on a base strip. More specifically, the method of this invention contemplates the step of applying a heating or a cooling fluid to selected longitudinal areas or zones of a ferrous strip as it emerges from a bath of molten metal in order to promote a uniform cooling pattern.

The apparatus of this invention contemplates at least one manifold extending transversely across the full width of the strip and communicating with a supply of heating or cooling fluid under pressure. The manifold is provided with a plurality of spaced apart nozzles, each of which is individually controllable, so that the quantity of heating or cooling fluid supplied to any selected longitudinal zone on the moving strip can be regulated.

DESCRIPTION OF THE PREFERRED EMBODIMENT According to present metallic coating practice, the strip to be coated is thoroughly cleaned and, while maintained in a protective atmosphere, passed into a bath of molten coating metal. The strip is then withdrawn from the bath of molten metal in a generally vertically upward path of travel.

Upon emergence from the bath, the strip will carry with it a quantity of molten metal. This molten metal adhering to the strip may be subjected to or controlled by a variety of well known finishing techniques, including exit rolls, jet finishing nozzles, and the like.

The method and apparatus of this invention relate to the treatment of the coated strip after it passes the finishing device or devices briefly noted above, and prior to the solidification of the molten coating on the ferrous strip.

Reference is again made to Hoover et al. Pat. 3,379,- 557 which teaches a method for the suppression of visible spangle by inducing substantially sub-macroscopic spangling by applying a multitude of closely spaced solidification nuclei. This patent further teaches the importance of applying the solidification nuclei at a rather precisely defined coating temperature; namely, a temperature just above the freezing point of the coating metal.

Turning first to FIG. 1, it is now known that as the strip bearing the molten coating metal passes the finishing devices, it begins to cool at the edges of the strip. This cooling gradually progresses toward the center of the strip and forms, in effect, an inverted parabolic cooling pattern. In FIG. 1, a portion of a strip indicated generally at is moving in a vertically upward path of travel. The portion of the strip 12 will include solidified coating metal, while a portion 14 will have still molten coating metal on the surface. The inverted parabolic line 16 defines the boundary between the solidified and molten portions of the coating metal.

With reference to the foregoing discussion, it will be apparent that it is extremely difficult to find a single horizontal line at which the solidification nuclei can be applied so that all portions of the coating metal are at the proper temperature.

By comparison, the desired cooling pattern is indicated schematically in FIG. 2. Again, a portion of a strip moving vertically upward is indicated at 10'. The portion of the strip 12' is covered by solidified coating metal, while the portion 14' contains still molten coating metal. The line of demarcation between the solidified and the molten metal is indicated at 16, and it will be seen that it approaches a straight line normal to the path of movement of the strip.

The method and apparatus of this invention can best be considered by reference to FIGS. 3 and 4 which show schematically two embodiments of the invention. Considering FIG. 3 first, the molten coating metal level in the bath is indicated at 18, and the strip indicated generally at 20 is moved by conventional means in a vertically upward path from the bath. In this particular embodiment, the finishing of the metallic coating is accomplished by the jet nozzle indicated schematically at 22. The rectangle 24 is a schematic illustration of the device which is utilized to apply the solidification nuclei consisting of a water solution of a salt to the molten coating. It will of course be understood that the device 24 will be vertically movable so as to permit variations in the point at which the solidification nuclei are applied to the strip.

The method of this invention contemplates the step of selectively heating and/or cooling desired longitudinal areas of a moving strip, so as to control the cooling pattern. The objective, of course, is to change from the normal cooling pattern set forth in FIG. 1 to the desired pattern shown in FIG. 2.

In FIG. 3, this is accomplished by means of the manifold 26 which will be connected to a suitable source of fluid (not shown) at a temperature substantially different from the temperature of the molten coating metal. The manifold 26 is provided wtih a main control valve 28.

It will also be seen that the manifold 26 is of a length to extend transversely across the entire strip width. On the side of the manifold facing the strip, it will be provided with a plurality of nozzles indicated schematically in FIG. 3 at 30a through 301'. Each of these nozzles is provided with an individual control indicated schematically at 31. Thus, when the main control valve is opened, the flow of fluid can be selectively controlled through each of the individual nozzles.

There are of course a great many variables which affect the cooling pattern of the molten metal. They include the gauge and width of the base strip, the weight of coating metal applied, as well as ambient temperature and other atmospheric conditions. All of these variables may require a different use of the apparatus just described.

For example, with a high bath temperature, heavy gauge strip and high ambient temperatures, it may be necessary to supply a cooling fluid such as air to the manifold 26. The compressed air normally is supplied to the control valves at a pressure of 25-100 p.s.i. and a temperature of 60-150 F. Under these circumstances, the nozzles at the center of the strip width (30d, 30c, and 30 would be opened to provide a greater cooling effect in the center of the strip. Progressing toward the edges, the nozzles would be adjusted to provide a continuously smaller flow of fluid onto the coating.

By comparison, a lower bath temperature, light gauge strip and low ambient temperature might require that a heating fluid such as hot combustion gases be supplied to the strip. The temperature of the combustion gases would be in the range of l00O-2500 F. depending on the volume of fuel supplied, efliciency of combustion and distance from the nozzle. Pressure would be slightly above atmospheric. In this case, the nozzles at the edge of the strip (30b and 3011) would be opened to a greater extent to provide a greater heating action at the edges of the strip. Progressing toward the center, the nozzles would be adjusted to provide a lesser flow of heating fluid to the strip.

In other words, the nozzle at each position on the strip will be adjusted as necessary to vary the temperature of that portion or longitudinal zone of the strip with respect to the other zones so as to create a straight line temperature differential across the strip.

FIG. 4 illustrates schematically a modification of the invention. Again, the strip is indicated generally at 20, the level of molten metal in the bath at 18, the jet finishing nozzle at 22, and the apparatus for applying solidification nuclei to the strip at 24.

According to this embodiment, two manifolds 32 and 34 are provided, each of which extends across the full width of the strip. The manifold 32 and 34 are provided respectively with the main control valves 36 and 38. The manifold 32 is provided with the individual nozzles 40a through 401', while the manifold 34 is provided with the individual nozzles 42a through 42i. Each of the nozzles 40a through 401 is provided with an individual control valve 41, while each of the nozzles 42a through 421' is provided with an individual control valve 43.

With this embodiment of the invention, a heating fluid will be supplied to one of the manifolds, and a cooling fluid will be supplied to the other. Thus, certain longitudinal portions of the strip could be cooled while other longitudinal portions are simultaneously being heated.

The method or mechanism for controlling the adjustment of the individual nozzle controls 31, or 41, or 43, does not, per se, form a part of this application. In the simplest case, each nozzle control valve could be manually set and/or adjusted. Operating experience would indicate normal settings for each valve under a variety of different conditions.

It is of course within the scope of this invention to provide automatic control for each of the nozzles. For example, radiation pyrometers or other remote sensing devices could be utilized, and these could be arranged to automatically control the output of the various nozzles. U.S. Pat. 3,307,968 issued on Mar. 7, 1967, in the name of Paul E. Schnedler, teaches a method and apparatus for the remote sensing of the reflectivity of a zinc coated strip. Devices of this type could certainly be utilized to control the individual nozzle valves of this invention.

It is believed that the foregoing constitutes a full and complete disclosure of this invention. Numerous modifications may be made by the skilled worker in the art without departing from its scope and spirit, and accordingly, no limitations are intended except as specifically set forth in the claims which follow.

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

1. In a process for producing a zinc coated ferrous strip having a spangle which is substantially invisible to the naked eye and which includes the steps of cleaning the surface of said strip to render it receptive to the molten zinc, passing said cleaned strip into a bath of said molten zinc, withdrawing said strip from said bath with a quantity of said molten zinc adhering thereto, finishing said molten zinc adhering to said strip, and thereafter inducing sub-macroscopic spangling by applying a multitude of solidification nuclei to said finished molten zinc adhering to said strip when said zinc is at a temperature just above the freezing point; the improved step of controlling the cooling of said finished molten zinc by applying a heating or a cooling fluidto selected longitudinal zones of said strip.

2. The method claimed in claim 1 wherein said heating or cooling fluid is applied to said strip in graduated quantities from the center to the edges thereof.

3. The method claimed in claim 2 wherein a greater quantity of said cooling fluid is applied to the center of said strip.

4. The method claimed in claim 2. wherein a greater quantity of said heating fluid is applied to the edges of said strip.

5. The method claimed in claim 2 wherein said heating and cooling fluids are each applied to different longitudinal zones of said strip.

6. The method claimed in claim 1 wherein said step of controlling the cooling of said finished molten zinc includes the steps of:

(a) providing a manifold extending transversely across the full width of said strip and communicating with a supply of fluid under pressure at a temperature substantially different from the temperature of said molten zinc;

(b) providing a plurality of spaced apart nozzles in said manifold; and

(c) providing individual valve means associated with each said nozzle for controlling the flow of said fluid under pressure therethrough.

References Cited UNITED STATES PATENTS 3,459,587 8/1969 Hunter et al 117-102 M 2,708,171 5/1955 Inglefield, Jr. 117-114 AX 2,967,114 1/1961 Mayhew 117-114 AX 3,379,557 4/1968 Hoover et a1. 117-114 AX 1,675,646 7/ 1928 Fitzgerald 117-119.2 X 3,260,577 7/1966 Mayhew 117-114 AX 3,307,968 3/ 1967 Schnedler 117-114 A FOREIGN PATENTS 714,915 9/1968 Belgium 117-102 M OTHER REFERENCES Butler, J. J., et al., The Development of Air Coating Control for Continuous Strip Galvanizing, paper at A.I.S.I. meeting, Nov. 12, 1969, pp. 1, 15, 17.

ALFRED L. LEAVITT, Primary Examiner J. R. BATTEN, JR., Assistant Examiner US. Cl. X.R.

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