US3383250A - Method for producing one side metallic coated strip - Google Patents

Description

' May 14, 1968 M. B. PIERSON' ET AL 3,383,250

METHOD FOR PRODUCING ONE SIDE METALLIC COATED STRIP Filed Oct. 7, 1964 INVENTORS MARVIN B. Pmzscm AND NOEL W. Pmzxs,

ATTORNEYS United States Patent 3,383,250 METHOD FOR PRODUCING ONE SIDE METALLIC COATED STRIP Marvin B. Pierson and Noel W. Parks, Middletown, Ohio,

assignors to Armco Steel Corporation, Middietown,

Ohio, a corporation of Ohio Filed Oct. 7, 1964, Ser. No. 402,125 2 Claims. (Cl. 148-635) ABSTRACT OF THE DISCLOSURE A method of producing a strip of metal having a metal coating on only one side thereof. Both sides of the metal strip are first cleaned. The metal strip is brought to coating temperature and one side only of the metal strip is then oxidized. This metal strip is maintained in a protective atmosphere at coating temperature as it is then moved into the coating bath. The coating metal adheres to the clean side of the metal strip but not to the oxidized side.

This invention resides in a method for applying an adherent metal coating to steel strip on one side only. The invention may readily be incorporated as a modification of convention-a1 continuous coating processes. It may also be accomplished in conjunction with a vertical open flame strip heating furnace as will be further described below. Coating of the strip will normally be accomplished by dipping it in a molten metal coating bath. The invention embraces the use of Zinc, aluminum or lead, or any hot dip coating metal.

In recent years there has been a growing demand for steel strip which has been galvanized on one side only. Such a product is quite useful because the uncoated side can be welded by spot welding techniques more easily, and has better painting characteristics than untreated galvanized surfaces. Methods which have been tried by others for the production of single side coatings have used the application of stop-off materials to one side of the strip to prevent wetting by the coating metal. And some methods have attempted simply to use the inking roll process, rather than dipping, to apply the coating to one side only.

The best prior art known to the inventors includes three patents and a knowledge of certain undesirable results which have occurred during other processes by which it was intended that steel would be coated. The Greene et al. Patent No. 2,894,850 shows the coating of one side of a steel sheet or strip with an alkali metal aluminate solution. The Siebert et al. Patent No. 3,104,993 teaches providing on only one side of a ferrous metal sheet a thin, continuous coating of a refractory metal oxide composition prepared in the form of an aqueous colloidal solution or suspension having an oxy-mineral acid dissolved therein. The Blay Patent No. 3,121,019 discloses coating one side of the strip that is to be dipped in molten zinc with a solution of one of the alkaline earth hydroxides which includes calcium, magnesium, strontium and barium.

And, as has been indicate-d above, there has been a phenomenon which has occurred during prior coating operations and which has heretofore resulted in poor coatings. It has been observed for some time that when oxygen has accidentally found its way into the coating system, as might occur when leaks develop in the furnace snout, or when furnace combustion tubes perforate or crack, oxide patches have formed on the strip to which the coating material did not adhere, in this manner producing an undesirable product.

It is, therefore, an important object of this invention to provide a method for coating steel strip on one side only by providing on the opposite side a continuous oxide layer throughout, prior to the time the strip is passed through the hot dip coating metal.

Another import-ant object of the invention is to provide a method for coating one side only for a steel strip, which means and method may be readily incorporated in steel coating lines.

Briefly stated, one method of practicing the invention is to mask that side of the steel strip which is not to be coated by oxidizing that side of the very edge without oxidizing the other side, which other side is to be coated by the hot dip coating metal.

One way in which the benefits of this invention may be achieved is in conjunction with a vertical open flame strip heating furnace such as that shown in the Klein et al. Patent No. 3,010,844 in which high intensity radiant heat is produced by open, cup-shaped burners of the Selas type. The gas and :air fed to these burners are precisely regulated and completely pre-mixed so that the products of combustion contain excess combustibles to the extent that the atmosphere is chemically reducing to steel strip at normal processing temperatures. In the present invention jets of air or oxygen are directed against one side of the heated strip near the bottom of the combustion zone. Oxygen contacting the hot strip oxidizes the strip surface on that side but any excess oxygen is engulfed in the relatively large volumes of reducing combustion gases of the furnace and is rendered reducing by combustion before it can contact and oxidize the opposite side of the strip. In the Klein et al. process, the strip is precleaned chemically so that the function of the open flame radiant heater primarily to rapidly heat the strip to coating temperature, or slightly above (in the neighborhood of about 1000 F.) 'in a protective atmosphere. This relatively low strip temperature is adequate because furnace treatment is not normally required for strip surface preparation and metallurgical treatment for strip mechanical properties is accomplished as a separate prior operation.

Following passage of the strip through the open flame heater, the strip travels through a closed connecting transfer chamber to the coating bath with the end of the chamber sealed in the molten coating metal. The chamber maintains proper strip temperature for coating and the chemically cleaned strip surface is protected with a 10W hydrogen content protective atmosphere. After leaving the strip heater the strip with one side oxidized and one side clean is not exposed to potent reducing condition by virtue of the low hydrogen content atmosphere of the transfer chamber and the low strip temperature. This condition is essential in the present invention. If the strip were exposed to potent reducing conditions (high hydrogen and high strip temperature) the oxidized surface would be reduced at least enough for :a quasi-adherent coating to be applied to the oxidized surface which would be impractical to remove. With the oxidized surface reaching the bath essentially unreduced the coating metal will not adhere at all, or only superficially adhere. Any coating metal that superficially adheres may be easily removed by wire brushing or the like, there is no true bonding of the coating to the steel strip on that side which has been processed as described.

These objects and statements regarding this invention will be amplified with reference to the accompanying drawing and the description to follow, which drawing consists of the single figure depicted:

The figure is a schematic illustration of the technique for oxidizing one side of strip in conjunction with a continuous galvanizing line such as shown in the Klein et al. Patent No. 3,010,844.

Referring now to the figure there is diagrammatically illustrated a continuous galvanizing line to which this invention has been applied. A coil of strip steel material to be coated is located at the input of the line. The strip 11 may then pass from the coil 10 through a series of compartments 12, 13, 14, 15 and 16 in Which the strip may successively be exposed to an alkali, a rinse, an acid, a rinse, and a drying means respectively. From the drying means 16 the strip 11 passes over a deflector roll 17 and then passes vertically downwardly through a furnace generally designated at 18. This furnace 18 is a vertical open flame strip heating furnace in which high intensity radiant heat is produced by open, cupshaped burners of the Selas type as generally indicated at 19.

Streams of air and combustible gas, from sources not shown, are conducted through conduits 20 and 21 to a gas-air ratio regulator 22, and the output from the regulator 22 is conducted through a conduit 23 to the inlet of the valve 24. The valve 24 is provided with an operating mechanism 25 which may be adjusted to establish a desired temperature in the chamber 26 of the furnace 18 and which functions responsively to the temperature of the strip material passing from the heating chamber of the furnace. For this latter purpose a radiation pyrometer or proximity thermocouple type of temperature sensing means 27 is located in the chamber 28 adjacent the path of the strip material coming from the furnace 18 and is connected to the valve operating mechanism 25 through a conductor 29. The burners 19 receive the carefully regulated mixture of gas and air from manifolds 30 and 31 through the valve 24 and feed conduit 32. The regulator 22 is adjusted to feed to the conduit 23 a composite stream of air and combustible gas of the proper ratio so that the gaseous products of combustion of the heating units or burners 19 are nonoxidizing to the strip.

The strip 11 passes through the heating chamber 26 and chamber 28 whereupon it passe around a deflector roll 33 and into the chamber 34 of the closed connecting transfer member 35. A roll 36 is located in the upper end of the transfer member for directing the strip 11 downwardly through the snout or delivery chute 37 into the bath of molten metal and around a sinker roll 38. The end of the chute 37 extends into the bath of molten metal 39 contained within the pot 40. The strip 11 then passes upwardly through a cooling section diagrammatically illustrated at 41, around a deflector roll 42, downwardly and around a roll 43, and then formed into a coil 44 at the exit end of the line. The cooling section 41 may comprise a conventional cooling tower not shown, and coiling devices may be located at the exit end of the line upon which the coil is formed.

The furnace 18 permits extremely close and accurate control of the fuel-air ratio so that one may get from it either a reducing potential or an oxidizing potential relative to steel strip. The burning of natural gas (CH and oxygen, for example, results in heat plus combustion products, which products are carbon monoxide (CD), plus carbon dioxide (CO plus water (H O), plus hydrogen (H and perhaps some excess CH If enough oxygen is utilized, then more CO and H 0 and excess 0 result, thus producing an oxidizing atmosphere. If less oxygen is used, then more CO and H result thus producing a reducing atmosphere. In the arrangement of this invention the furnace 18 and burners 19 within the chamber 26 are controlled to produce products of combustion containing excess combustibles which are reducing to steel strip.

The apparatus heretofore described with specific reference to the figure is conventional. The crux of the invention with respect to this arrangement lies in the position of means generally indicated at 45 for providing jets of air or oxygen directed against one side of the heated strip near the bottom of the combustion zone 26. The delivery end 46 of the means 45 may take the place of one or more of the cupshaped burner units 19. The air, oxygen or l oxidizing flame delivered from the member 46 can be directed more or less on the center portion of the strip 11 with complete coverage of the strip obtained by Washing action of the deflected oxidizing gases.

The oxygen which contacts the hot strip via the member 46 near the exit end of the chamber 26 oxidizes the strip surface on the side contacted but such oxygen is engulfed in the relatively large volumes of reducing combustion gases emanating from the burners 19 with the result that the resulting atmosphere is rendered reducing by combustion before the injected oxygen can contact and oxidize the opposite side of the strip 11.

The furnace 18' may be operated to produce relatively low strip temperatures, that is, in the neighborhood of 800 F, to 1000 F. And, although normal operation of the furnace 18 might tend to prevent the formation of oxides, in this invention the formation of an oxide layer on the side 11a of the strip 11 is positively insured by specifically directing an oxidizing gas from the source 45 and conduit 46 against such side 11a. The low hydrogen protective gas supplied at 52 is insufficient to reduce the oxide on side 11a.

After the strip 11 has had its one side 11a oxidized to the edges as just described, the strip passes through the closed connecting transfer chamber 35 and through the delivery chute or snout 37 into the molten coating metal 39 contained in the pot 40. Although this coating metal will generally be zinc, it may also be aluminum, lead or other suitable coating materials. This coating metal will bond to the clean side of the strip 11 but it will not bond to the side 11a which has been oxidized as described. It is possible, however, to the extent that the oxide layer may have become very slightly reduced between the time it left the furnace 18 and entered the bath 39, that there may be some superficial adherence of the coating metal to such slightly reduced oxide layer; this, however, is not a true bonding of the coating metal to the oxidized side 110. Any such adherence of the coating metal to the oxidized layer may easily be removed by a simple rotary brush or the like such as is shown at 47.

It has earlier been mentioned that difficulties formerly occured during operations wherein it was desired to effect a bonding between clean steel strip and the coating metal when portions of the clean strip became accidentally oxidized, It was observed that these accidentally oxidized portions would prohibit correct bonding of the coating metal to the strip. This at that time produced an inferior strip. Broadly considered, therefore, a feature of this invention resides in the conversion of these prior, accidental and objectionable occurrences into a controlled process producing desirable, improved results.

After the strip 11 emerges from the coating pot 40, with the coating metal bonded to the side 11b of the strip 11, but not to the side 11a, it may or may not be necessary to remove the oxide layer from the side 11a. Ordinarily it is desirable to remove such oxide layer and this may easily be accomplished by conventional (chemical and mechanical) practices.

The invention resides in having the oxide layer on side 11a extend from edge to edge without any oxidizing of the opposite side, said oxide layer to be intact as the strip enters the coating pot. Direct impingement of air or oxygen from the arrangement 45, 46 on the very edges of the strip 11 is not required. It is sutficient that the oxidizing media be directed more or less on the center portion of the strip with complete coverage of the strip being obtained by the washing action of the deflected oxidizing gases. This removes the possible hazard of oxidizing the edges on the opposite side of the strip, that is, the side which is to be coated. The trick is to keep an excess of reducing gases washing over the opposite side so that it will immediately combine with any oxygen which might overflow from the side 11a so that said oxide layer is restricted to the side 1111.

It should be understood "that in the broad application of the principles of this invention it does not matter how the strip to be oxidized on the one side is prepared for coating. Thus, for example, gases may sometimes be employed to oxidize and/or vaporize surface contaminants and to reduce the strip in order to prepare it for bonding, while at other times chemical preparations may be employed for this purpose. It is important only that the strip be properly cleaned and then, more or less at the last moment, one side thereof be oxidized without oxidizing any of the other side.

With respect to the oxide layer provided at 11a by the means and methods of this invention, it is pointed out that only a light oxide is required, one which may be described as blue to straw color.

It is believed that the foregoing constitutes a complete description of this invention. It will be understood that modifications of this invention may be made by those skilled in the art without departing from the scope and spirit of the invention. Also, while the invention has been described in terms of particular methods and means, the invention is not to be restricted specifically to such methods and means except in so far as they are specifically set forth in the claims to follow.

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

1. A method of producing a steel strip having single side coating which includes the steps of preparing one side of said strip to receive said coating, applying a large volume of reducing gases to said one side of said strip While simultaneously applying relatively small amounts of oxidizing gases to the other side of said strip while said stn'p is at a reactive temperature for oxidation whereby to form an oxide layer thereon without forming any oxide on said one side, bringing said strip to coating temperature and transferring said strip to a bath of molten metal under such conditions as to maintain said oxide layer, and dipping said strip in said bath of molten metal whereby to form a coating thereof on said one side only.

2. A method of producing a coated steel strip having one side to which another metal adheres well when the strip is passed into a coating pot containing sucli metal in molten condition, and an opposite side to which said metal does not adhere, which method comprises;

(a) preparing the strip so that a metal coating will adhere thereto;

(b) applying an oxidizing gas to one side of the strip While said strip is at a reactive temperature for oxidation so as to form an oxide thereon and simultaneously providing sufiicient reducing gas on the opposite side of said strip to prevent any oxidizing of said opposite side of said strip even after anyiexcess of said oxidizing gas combines with the said'reducing gas whereby to produce an intermediate product comprising steel strip having an oxide on said one side and a reduced surface on said opposite side; and

(c) bringing said strip to coating temperature and passing said intermediate product (the oxidized-reduced steel strip) into a coating pot containing molten metal before any substantial alterations in the characteristics of said intermediate product occur, whereby the molten metal adheres to the reduced opposite side and not to the oxidized one side, thus achieving a single side coating of said steel strip.

References Cited UNITED STATES PATENTS 222,655 12/1879 Breeding 117-94 X 1,231,285 6/1917 Mueller 117-51 2,197,622 4/ 1940 Sendzimir 117-5 1 2,570,906 10/1951 Alferielf 117-51 3,010,844 11/1961 Klein et al 117-114 X 3,104,993 9/1963 Sievert et al. 117-5.5 X 3,177,085 4/ 1965 Adams 117-5.5

RALPH S. KENDALL, Primary Examiner.

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