US3511686A

US3511686A - Method for annealing and coating metal strip

Info

Publication number: US3511686A
Application number: US701526A
Authority: US
Inventors: David A Withrow
Original assignee: Production Machinery Corp
Current assignee: Production Machinery Corp
Priority date: 1968-01-11
Filing date: 1968-01-11
Publication date: 1970-05-12
Anticipated expiration: 1987-05-12
Also published as: BE720353A; JPS5010251B1; FR1581354A

Description

May 12, 1970 D. A. WITHROW 3,51 METHOD FOR ANNEALING AND COATING METAL STRIP Filed Jan. 11. 1968 INVENTOR. DAVID A. WITHROW United States Patent 3,511,686 METHOD FOR ANNEALING AND COATING METAL STRIP David A. Withrow, Willoughby, Ohio, assignor to Production Machinery Corporation, Mentor, Ohio, a corporation of Ohio Continuation-impart of application Ser. No. 381,976, July 13, 1964. This application Jan. 11, 1968, Ser. No. 701,526

Int. Cl. C230 1/02, 1/04, 1/08 US. Cl. 11751 4 Claims ABSTRACT OF THE DISCLOSURE There is provided a method of coating a ferrous metal strip with a metal, such as zinc, tin, aluminum, and alloys thereof. While the strip is moving, it is first heated in a molten metal bath to a temperature below the annealing temperature of the strip. Then, the strip is heated to the annealing temperature While in an oxidizing atmosphere to form an oxide on the strip; quenched in a bath of molten metal; and, then passed into a reducing atmosphere. This reduces the oxide on the strip and prepares the strip for subsequent coating. While in the reducing atmosphere, the strip temperature is regulated to the proper temperature for subsequent coating.

This application is a continuation-in-part application of my copending application Ser. No. 381,976, filed July 13, 1964 and now abandoned.

This invention pertains to the art of coating metal strip and more particularly to a method and apparatus for both annealing and coating a metal strip.

This invention is particularly applicable to annealing and coating a steel, or ferrous, strip with a zinc base alloy, which process is generally known as galvanizing, and it will be described with particular reference thereto; however, it is to be appreciated that the invention has much broader applications and may be used for applying various metal coatings onto strips having a variety of compositions. For instance, the method and apparatus may be utilized for tin plating, terne plating and aluminizing, to name only a few of the other coating or plating processes for which the invention is applicable.

For many years, steel strip has been coated with a zinc base alloy to provide a protective outer surface. The apparatus for applying the alloy generally included a device for unreeling the strip from a coil and a device for cleaning the surface of the strip preparatory to the actual coating operation. The clean strip was then fluxed so that the previously cleaned surface would not oxidize before the strip was drawn through a molten bath of zinc base alloy, in which bath the strip was coated with the alloy.

An improved process for galvanizing strip has been generally adopted by the trade. In this process, the clean strip is first oxidized with an oxidizing medium, such as steam, to form a thin oxide film of controlled characteristics on the surface of the strip. The strip is then passed through a reducing atmosphere which reduces the oxide film on the surface of the strip to form a clean, etched metal surface. The essence of this process is the reduction of the oxide on the surface of the strip to produce an etched surface which enhances the adhesive or bonding characteristics of the strip surface. In this manner, the etched metal surface on the strip provides a rigid bond between the zinc base alloy being applied to the strip and the strip itself. The present invention is directed toward a method and apparatus utilizing either a combined oxidation-reduction procedure or a reduction procedure alone for forming a superior surface onto which the zinc base alloy adheres and more particularly to an 'ice improvement over known processes and apparatus for accomplishing this type of strip coating or galvanizing.

In many cases, before the strip can be coated, the strip must be annealed for metallurgical purposes and to increase the ductility and relieve the internal stresses caused by previous rolling and forming of the strip. If the strip is annealed in a remote location and then coiled, there is a possibility that a substantial amount of random oxide will form on the surface of the strip which oxide is detrimental to the oxidation-reduction or reduction galvanizing procedure outlined above. More specifically, the uncontrolled oxidation of the annealed strip forms oxide of an uneven thickness, and causes a relatively heavy accumulation of oxide on the strip. With this uneven and heavy oxidation, when the strip is subjected to a reducing atmosphere, the surface may have either a spongy mass of reduced metal or unreduced oxide or a combination of spongy mass and oxide. In all of these cases, the bond between the zinc base alloy and the strip may be imperfect, and subsequent flexing of the strip may cause flaking of the coating from the strip. For this reason, in the oxidation-reduction process for galvanizing the strip, it is generally necessary to pickle previously annealed strip before the strip enters the coating apparatus.

It has been suggested that the annealing of the strip preparatory to coating could be accomplished in a single apparatus wherein the strip was annealed, the surface was oxidized, the surface was then reduced and then the strip was introduced into a molten bath of the zinc base alloy. This suggestion has not been completely successful because the annealing apparatus for the strip has heretofore included apparatus for heating the strip to the annealing temperature with a gas medium or with radiant heaters. Consequently, it was necessary to provide a relatively long heating furnace in order to raise the strip to the annealing temperature which may be in the range of 1500 F.-1700 F. when the strip is steel or other ferrous base metals. The furnace could be reduced in length by decreasing the speed of the strip; however, this is not economical because the rest of the apparatus can handle the strip at a relatively high velocity. The inefiiciency and/or the tremendously long length of the equipment necessary for a combined annealing and coating installation have been the primary reasons for the trade not completely adopting a combined annealing and coating installation for processing strip metal. In addition, in such a suggested apparatus the strip would become overheated if the apparatus were shut down for a very short time. And when the strip was cooled in such an apparatus, there was no etficient way of transferring the heat dissipated from the strip during cooling back to the heating portion of the apparatus. This latter factor further decreased the etficiency of such an apparatus.

These and other disadvantages have been completely overcome by the present invention which is directed toward a combined annealing and coating method and apparatus for metal strip which requires a relatively small apparatus that is highly efiicient in operation.

In accordance with the present invention there is provided a method of coating a metal strip with a dissimilar metal which method comprises the steps of heating the strip to a temperature at least approaching the annealing temperature of the strip by subjecting the strip to a molten metal bath; cooling the strip to a temperature below the annealing temperature of the strip and above a predetermined coating temperature by subjecting the strip to a molten metal bath; maintaining the cooled strip in a reducing atmosphere and then passing the strip through a molten bath of the dissimilar metal before it is exposed to ambient conditions.

In accordance with another aspect of the present invention, a method as defined above includes the step of oxidizing the surface of the strip after the strip has been heated and before it is cooled. The oxide formed in this manner is subsequently removed by the reducing atmosphere in which the strip is maintained prior to passing into the molten coating bath. Removal of the oxide leaves a layer of base metal on the surface of the strip, which layer is beneficial in forming the bond between the strip and the coating metal. In addition, the oxide on the surface of the strip prevents wetting of the strip as it is being cooled by the molten metal so that the molten metal is not pulled into the coating bath.

In accordance with another aspect of the present invention, there is provided an apparatus for coating :1 metal strip wtih a dissimilar metal. This apparatus comprises a first molten bath of a first metal at a temperature below the annealing temperature of the strip; a second molten bath of the first metal, the second bath being maintained at a temperature below the annealing temperature of the strip; an enclosed chamber joining the first and second bath; means for filling the chamber with a controlled oxidizing or reducing atmosphere; a third molten bath of the dissimilar metal; a second enclosed chamber joining the second and third, baths, means for filling the second chamber with a reducing atmosphere; and, strip drive means for propelling the strip successively through the first bath, the first chamber, the second bath, the second chamber and, then, the third bath.

The primary object of the present invention is the provision of a method and apparatus for annealing and coating a metal strip with a dissimilar metal which method and apparatus are economical in operation and require a relatively small floor area.

Another object of the present invention is the provision of a method and apparatus for annealing and coating a metal strip with a dissimilar metal which method and apparatus includes annealing the strip immediately adjacent the coating apparatus.

Still another object of the present invention is the provision of a method and apparatus for annealing a metal strip with a dissimilar metal which method and apparatus utilize a molten metal for heating and cooling the strip during the annealing thereof.

These and other objects and advantages will become apparent from the following description used to illustrate the preferred embodiment of the invention as read in connection with the accompanying drawing in which the single figure is a schematic view showing the basic operating characteristics of the present invention.

Referring now to the drawing wherein the showings are for the purpose of illustrating the preferred embodiment of the invention only and not for the purpose of limiting same, the figure shows, schematically, an apparatus A for annealing and coating a steel strip B with a zinc base alloy which coating process is generally referred to as galvanizing. It is appreciated that the strip may be formed from various metals and that steel strip is used only for illustrative purposes. The coating metal may be any of a variety of metals, such as tin and aluminum or aluminum alloys. Essentially, the apparatus A includes an annealing section and a coating section 12.

Referring now more particularly to the annealing section 10 of the apparatus, there is provided a preheat bath 20 filled with a molten metal having a low melting point, such as eutectic lead-bismuth. It is appreciated that other molten metals may be used without departing from the intended spirit and scope of the present invention, such as sodium, potassium, mercury or alloys thereof. The preheat bath 20 includes an entrant channel 22 and an exit channel 24 between which there is located a guide roll 26 so that strip B can pass through channel 22, around roll 26 and upwardly through the exit channel 24. The temperature of the metal within channel 22 increases from the entrant end to the bottom portion and the temperature of the strip increases as it passes downwardly toward the roll 26 because the temperature of the metal is greater than the temperature of the strip. The temperature of the strip continues to increase as the strip passes upwardly through the channel 24. In accordance with the preferred embodiment of the present invention, the steel strip is heated. to a temperature of approximately l200 F.; however, the temperature range as the strip leaves the channel 24 may be anywhere between 600 F. and i200 F. according to the particular strip being annealed and the amount of heating after leaving the channel 24. In essence, the strip should be approximately 200 F. to 400 F. below the annealing temperature for the metal forming the strip. In order to increase the heat of the molten metal within channel 24 there is provided, in accordance with the illustrated embodiment of the invention, a heater 28 adapted to direct heat into the molten metal within the channel. The level of he molten metal within preheat bath 20 is indicated at 30 which is slightly above the highest end of inclined wall 32 so that the molten metal may flow over the inclined wall from the quench bath 40 to be hereinafter described in detail.

Referring now to the quench bath 40, there is provided an entrant channel 42, an exit channel 44 and an intermediate guide roll 46 so that the strip B can move downwardly through the channel 42 around roll 46 and upwardly through channel 44. The quench bath 40 is filled with the same molten metal which fills the preheat bath 20 and this metal circulates from the quench bath over the inclined wall 32 as previously described. The upper portion of channel 44 is provided with a conduit 50 to direct the molten metal to the exit end of channel 44 from conduit 52 which is connected to the entrant end of channel 22.

Between conduits 50-, 52 there is provided an appropriate pump 54 which may take a variety of structural embodiments. By providing the conduits 50, 52 and pump 54, molten metal can be continuously circulated b the pump through the preheat bath 20 and the quench bath 40. The heat energy transferred by the heated strip to the molten metal within bath 40- increases the temperature of the molten metal in bath 40; By pumping this heated metal over the wall 32 to the end of channel 22, the heat released by the strip in the quench bath is utilized for heating the strip which enters the ichannel 22. This provides a highly eificient thermal system for heating and cooling the strip B with the heat released by the strip during cooling being used to heat the strip during the heating cycle.

Since the heat transfer between molten metal and the metal strip is 30-60 times greater than the heat transfer between a gaseous substance and a similar strip, the strip B is heated rapidly. It has been found that the strip may be heated within two seconds which is a substantial advance over known apparatus in a similar annealing and coating installation. Between the channels 24 and 42 there is provided an enclosed chamber 60 having a gas inlet 62 for introducing an atmosphere into the chamber. When the strip is being galvanized, it is often advisable to introduce a reducing atmosphere, such as hydrogen, ammonia, HCl, or H 50 vapor, into chamber 60. In this manner, any oxides on the surface of the strip will be reduced or the reduction of these surface oxides will be started for completion later in the processing of the strip, in a manner to be hereinafter explained in detail. When other metals are being applied to the surface of the strip by apparatus A, or in some situations where the apparatus is used for galvanizing, the atmosphere introduced into chamber 60 is an oxidizing medium, such as air or steam. The oxidizing medium forms a thin uniform oxidized film on the surface of the strip as the strip passes over guide rolls 64, 66 within the enclosed chamber 60. This thin uniform oxidized film is basically an oxide of the base metal forming the strip and it forms uniformly thereon during rapid passage of the strip through chamber 60.

In order to heat the strip, in accordance with the illustrated embodiment of the invention, a transformer primary winding 68 is secured within chamber 60 around laminated core 68a having windows 68b for the passage of the strip B as it makes a loop within chamber 60. The primary winding 68 is electrically connected onto an appropriate source of alternating current, which is schemat ically represented as generator 69. Since the molten metal within

baths

20, 40 is conductive, the strip and the molten metal in conduits 50, 52 form an electrically conductive path which acts as a secondary of a transformer so that energization of the primary winding 68 causes a current flow through the strip and through the metal in the conduits, as explained. This current flow causes an 1 R heating of the strip to raise the temperature of the strip to the desired annealing temperature while it passes through the atmosphere filled chamber 60. It is appreciated that the heat generated within the strip is utilized for heating the molten metal within

baths

20, 40. The heater 28 is utilized only when the heated strip does not raise the temperature of the molten metal within bath to the desired level. Although this transformer heating arrangement is shown in the illustrated embodiment of the invention, it is appreciated that an induction heating coil could be utilized within chamber 60 to perform the heating function without departing from the intended spirit and scope of the present invention.

The uniform thin film of oxide on the surface of strip B, when an oxidizing atmosphere is used in chamber 60, prevents wetting of the strip as it passes through the quench bath 40 so that the metal of the quench bath is not pulled therefrom as the strip leaves the channel 44 in its upward path. When a reducing atmosphere is used in chamber 60, during galvanizing, a certain amount of metal may be pulled from the quench bath by the moving strip; however, because the speed of the strip is generally slower when galvanizing, as opposed to other plating, and because lead bismuth drag out is not detrimental under normal conditions in galvanizing, this metal pulled from the quench bath is not harmful to the operation of the apparatus.

The temperature of the strip as it leaves the channel 44 is determined by the temperature required for the coating operation and, therefore, the temperature is varied when different metals are to be coated onto the strip. It has been found that this temperature will vary between 300 F. and 1,000 F. Tin plate requires a heating of the strip to a temperature in the range of 620 F. to 680 F., galvanizing requires heating the strip to a temperature of 850 F. to 920 F. and aluminizing of the strip requires heating of the strip to 1,200 F. to 1,280 F. In the last mentioned process, it may be necessary to heat the strip after it leaves the channel 44 during the annealing operation since it will probably leave the bath at a maximum temperature of about 1,000 F. The temperature of the molten metal at the exit end of channel 44 is controlled somewhat by the amount of circulation through pump 54. As the circulation increases, the temperature of the exit end of channel 44 decreases.

Referring now to the coating section 12 of apparatus A, there is provided an enclosed chamber 70 having an inlet 72 for introducing a reducing atmosphere into the chamber 70. This reducing atmosphere may take a variety of forms; however, in accordance with the preferred embodiment of the invention hydrogen, gaseous cracked ammonia, HCl, or H SO vapor is used in the chamber as the reducing atmosphere. When the strip passes through the chamber 70 around guide rolls 74, 76 the uniform thin surface oxide formed in chamber 60, when an oxidizing atmosphere is used in the chamber 60, is reduced by the atmosphere within chamber 70. This leaves an etched metal surface which forms a superior bond when subsequently coated with a dissimilar metal.

Of course, the chamber 70 is well insulated from its surroundings so that the temperature of the strip does not drop substantially while passing through the chamber. If a reducing atmosphere is used in chamber 60, which is the situation generally during galvanizing and other coating processes in apparatus A wherein the temperature of the strip in channel 60 is sufiiciently high and the speed of the strip moving through the chamber is sufficiently low to accomplish complete oxide reduction by using only a reducing atmosphere in chamber 60, the reduction atmosphere in chamber 70 augments the oxide reduction effected in chamber 60. In certain instances, complete reduction takes place in chamber 60, and chamber 70 only maintains the oxide-free surface.

Chamber 70 includes a final reducing channel 78 having an inlet 80, an outlet 82 and a pump 84 so that the atmosphere within chamber 70 may be pumped from the entrant side of the chamber to the final reducing channel 78 so that complete reduction of the oxide on the surface of the strip is accomplished before the strip B passes through extension 86. This extension protrudes downwardly into coating bath so that there is no loss of reducing atmosphere from the chamber 70. Consequently, the chamber 70 forms a continuous reducing atmosphere for the strip as it passes from channel 44 to the coating bath 90.

In accordance with the illustrated embodiment of the present invention, the bath 90 is a bath of zinc base alloy to form a galvanized surface on the strip B. The molten metal of bath 90 is held within furnace 92 having an appropriate heater 94 and a guide roll 96 so that the strip may be passed through the molten metal bath 90 and outwardly therefrom through rolls 98.

From the above description of the present invention it is appreciated that the complete apparatus A will require a relatively small amount of floor space and will economically anneal the strip and simultaneously coat the annealed strip with the desired surface metal. This is a substantial advance over any known annealing and coating apparatus.

When steel strip is being galvanized, and as an example, the annealing temperature is in the range of 1200 F. to 1700 F., and the strip is preheated by molten metal in channel 24 having an exit temperature in the general range of 1000 F. to 1200 F. The molten metal in channels 42, 44 has a temperature in the general range of 800 F. to 1000 F. at the exit position.

Having thus described my invention, I claim:

1. A method of coating a ferrous metal strip with a dissimilar metal selected from the class consisting of zinc, tin, aluminum and alloys thereof, said method comprising the steps of: moving said strip longitudinally along a given path, preheating said strip to a temperature below the annealing temperature of said strip by passing said strip through a molten bath of metal selected from the class consisting of lead, bismuth, potassium, sodium, mercury, and alloys thereof, at a temperature in the range of 200 F. to 400 F. below said annealing temperature; electrically heating said strip to the annealing temperature in an enclosed chamber filled with an oxidizing atmos phere so as to anneal said strip and form an oxide coating thereon; then, quenching said strip by passing said strip through a molten bath of metal selected from the class consisting of lead, bismuth, potassium, sodium, mercury, and alloys thereof, at a temperature below said annealing temperature of said strip; passing said strip through a second enclosed chamber filled with a reducing atmosphere to reduce the oxides; adjusting the temperature of said strip in said second enclosed chamber to a' temperature above the molten temperature of said dissimilar metal; and, then passing said strip through a molten bath of said dissimilar metal.

2. A method of coating a ferrous metal strip with a zinc base metal, said method comprising the steps of: moving said strip longitudinally along a given path;

preheating said strip to a temperature below the annealing temperature of said strip by passing said strip through a molten bath of metal selected from the class consisting of lead, bismuth, potassium, sodium, mercury, and alloys thereof, at a temperature in the range of 200 F. to 400 F. below said annealing temperature; electrically heating said strip to the annealing temperature in an enclosed chamber filled with an oxidizing atmosphere so as to anneal said strip and form an oxide coating thereon; then, quenching said strip by passing said strip through a molten bath of metal selected from the class consisting of lead, bismuth, potassium, sodium, mercury, and alloys thereof, at a temperature below said annealing temperature of said strip; passing said strip through a second enclosed chamber filled with a reducing atmosphere to reduce the oxides; adjusting the temperature of said strip in said second enclosed chamber to a temperature in the range of approximately 850 F. to 920 F.; and, then passing said strip through a molten bath of said zinc base metal.

3. A method of coating a ferrous metal strip with an aluminum base metal, said method comprising the steps of: moving said strip longitudinally along a given path; preheating said strip to a temperature below the annealing temperature of said strip by passing said strip through a molten bath of metal selected from the class consisting of lead, bismuth, potassium, sodium, mercury, and alloys thereof, at a temperature in the range of 200 F. to 400 F. below said annealing temperature; electrically heating said strip to the annealing temperature in an enclosed chamber filled with an oxidizing atmosphere so as to anneal said strip and form an oxide coating thereon; then, quenching said strip by passing said strip through a molten bath of metal selected from the class consisting of lead, bismuth, potassium, sodium, mercury, and alloys thereof, at a temperature below said annealing temperature of said strip; passing said strip through a second enclosed chamber filled with a reducing atmosphere to reduce the oxides; adjusting the temperature of said strip in said second enclosed chamber to a temperature in the range of approximately 1200 F. to 1280" F.; and, then passing said strip through a molten bath of said aluminum base metal.

4. A method of coating a ferrous metal strip with a tin base metal, said method comprising the steps of: moving said strip longitudinally along a given path; preheating said strip to a temperature below the annealing temperature of said strip by passing said strip through a molten bath of metal selected from the class consisting of lead, bismuth, potassium, sodium, mercury, and alloys thereof, at a temperature in the range of 200 F. to 400 F. below said annealing temperature; electrically heating said strip to the annealing temperature in an enclosed chamber filled with an oxidizing atmosphere so as to anneal said strip and form an oxide coating thereon; then, quenching said strip by passing said strip through a molten bath of metal selected from the class consisting of lead, bismuth, potassium, sodium, mercury, and alloys thereof, at a temperature below said annealing temperature of said strip; passing said strip through a second enclosed chamber filled with a reducing atmosphere to reduce the oxides; adjusting the temperature of said strip in said second enclosed chamber to a temperature in the range of approximately 620 F. to 680 F. and, then passing said strip through a molten bath of said tin base metal.

References Cited UNITED STATES PATENTS 2,019,445 10/ 1935 Crapo. 2,110,893 3/1938 Sendzimir 11751 X 2,111,826 3/ 1938 Waltman et a1. 2,797,173 6/ 1957 Keller 117-51 2,926,103 2/1960 Brick 11751 FOREIGN PATENTS 23 1,781 2/ 1964 Austria. 702,976 2/ 1941 Germany.

ALFRED L. LEAVITT, Primary Examiner J. R. BA'ITEN, JR., Assistant Examiner US. Cl. X.R. 117114; 118-429