US2224578A - Method and apparatus for coating strip or the like - Google Patents

Description

Dec. 10, 1940. R J N ETAL 2,224,578

METHOD AND APPARATUS FOR COATING STRIP OR THE LIKE Filed Feb. 16, 1939 2 Sheets-Sheet 1 Q INVENTORS I Raymond J. Wean 8r Marry W. The/'ss ma Dec. 10, 1940. R. J. WEAN EI'AL METHOD AND APPARATUS FOR COATING STRIP OR THE LIKE Filed Feb. 16, 1939 2 Sheets-Sheet 2 INVENTORS Raymond J. Wean 8:

rry W The omnwdooo Patented Dec. 10,1940

UNITED STATES PATENT OFFICE Raymond J. Wean, Warren, and Barry W. Theiss, Louisville, Ohio; said Wean assignor to The Wean Engineering Company,

Inc., Warren,

Ohio, a corporation of Ohio, and said Theiss assignor to Continental Steel Corporation,

Kokomo, Ind., a corporation of Indiana 11 Claims.

This invention relates to the application of a protective coating to a metallic base. Specifically, the invention concerns the coating of a steel base with zinc by passing it through a molten bath .of the latter.

It is well recognized that the zinc coating carried by galvanized sheets and wire does not adhere tightly to the steel base and is subject to flaking off on severe deformation. After considerable study and investigation, we have concluded that this is the result of the formation of an excessively thick layer of zinc-iron alloy next to the base metal during the galvanizing process. Our study indicates that this thick alloy layer forms on the base during its passage through portions of the spelter bath in which the percentage of iron present therein is substantial, i. e., 1% or above.

It has been considered essential heretofore to immerse the ferrous base in the spelter bath for a substantial length of time in order to bring the base up to the alloying temperature. In the coating of wire, for example, baths as long as 36 ft. have been employed. Immersion of the base in the spelter for substantial periods of time naturally increases the rate of dross formation and increases the tendency toward the formation of an excessively thick layer of zinc-iron alloy next to the surface of the ferrous base. The dross is an alloy of zinc and iron, the initially small, floating particles .of which agglomerate and eventually settle to the bottom of the bath.

The formation of zinc dross is objectionable not only from the standpoint of the quality of the product but because it increases the cost of manufacture. According to the present practice, the dross formed in a sheet-coating bath may amount to as much as 14% of the total zinc used. In a wire-coating bath, the percentage is even higher, viz., 22% to 24%, and as much as 16% to 18% of the zinc used may be lost as skimmings. While dross and skimmings have some reclamation value, it is .only a fraction of the cost of the zinc.

In order to avoid the objections to the conventional galvanizing method mentioned above, the so-called lead-zinc process has been developed. In this process a relatively shallow bath of molten zinc is floated on a deeper bath of molten lead. A flux box extends downwardly through the spelter into the lead and the sheet steel, after passing through the flux box, initially enters the lead bath. The immersion of the material in the lead bath preheats it but the lead does not "wet or alloy with the base. The

Application February 16, 1939, Serial No. 256,684

steel is finally coated as it passes from the lead bath through the spelter bath on emerging from the galvanizing pot or kettle. sults in a satisfactory product for the first few days of a weeks operations. Because of the extensive area of contact between the molten lead and zinc, however, the lead bath absorbs zinc to the saturation point and the flux, such as ammonium chloride, floating on the zinc tends to combine with the latter to a slight extent, forming zinc-ammonium chloride. In addition, dross forms at the bottom of the spelter bath. The combination of these circumstances, viz., the formation of zinc-ammonium chloride and the affinity of the zinc for the dross through which the sheets pass, causes black spots and streaks on the finished product, beginning about the middle of a weeks operations.

These defects in the finished product are sometimes referred to as flux spots pulling through the metal, but a more correct explanation appears to be that the flux is no longer in proper condition because of the formation of zinc-ammonium chloride and the same is true of the lead bath because of absorption of zinc. Regardless of the cause, the result is that the coating on the finished sheets is not uniform throughout the area thereof.

Galvanized material as now produced is subject to the further objection that if given a heat treatment after emerging from the spelter bath, to produce a dullor matte surface thereon, the coating rapidly crumbles or is converted to a powder form in which it has practically no adherence to the base.

We have invented a novel method and apparatus for applying a protective coating to a metallic base which overcomes the aforementioned objections and produces a protective coating which adheres tightly to the base even after severe deformation. In a preferred practice of the invention, we pass the base metal through a long pre-heating bath of molten lead, for example, and withdraw it therefrom through a bath of molten zinc. We so contrive, however, that the base is immersed in zinc only for a relatively short time and that the zinc and lead are in contact over only a small area. The zinc and lead baths may be enclosed and a non-oxidizing atmosphere may be maintained above their surfaces. As a result, we are able to produce a coat ed product which is free from dross spots and the excessively thick layer of zinc-iron alloy adjacent the surface of the base. The formation of dross in the zinc bath is practically elimi- This process renated because of the short time of immersion.

The formation of oxides in the form of skimmings on the zinc and lead bath is prevented by the neutral atmosphere maintained above the surfaces thereof. The cost of the operation is reduced by limiting the amount of dross and skimmings formed. The finished product, furthermore, has a small percentage of lead in the coating thereof which is desirable. It is also characterized by a high luster but the apparatus is well adapted to provisions for heat treatment subsequent to coating to produce a matte surface. When so treated, however, the coating is not subject to crumbling or conversion to powder form but retains its tight adherence to the base for longperiods of time.

By reducing the area of contact between the zinc and the lead to a minimum we prolong the period of operation which is possible without absorption of zinc by the lead bath to the saturation point.

The apparatus which we employ includes in addition to the usual pot or kettle for containing the molten metal, a settling tank for the zinc dross and a battle for confining the layer of zinc floating on the lead to a small portion of the lead bath adjacent the point at which the sheets emerge therefrom. We also provide separate heating means for the zinc and lead by which certain advantages hereinafter specified are obtained. We also provide means for positively withdrawing dross from the point at which the sheets pass through the spelter bath.

The following detailed description of the invention refers to the accompanying drawings illustrating the preferred embodiment and practice above mentioned. In the drawings:

Fig. 1 is a longitudinal section, largely diagram- 40 matic, through a sheet-coating apparatus for carrying out the invention;

Fig. 2 is a partial plan view with parts omitted; Fig. 3 is a partial transverse section taken along the line III-III of Fig. 2;

45 Fig. 4 is a central, longitudinal section through a strip-coating apparatus embodying the invention;

Fig. 5 is a transverse sectional view taken substantially along the plane of line V-V of Fig. 4; 50 and Fig. 6 is a partial sectional view similar to Fig.

1 showing a wire-coating apparatus.

Referring now in detail to the drawings, and for the present to Figs. 1 to 3, a galvanizing ket- 55 tle i0 is disposed in a brick setting ll. Burners l2 mounted in the setting discharge hot combustion gases around the kettle and through passages l3 along the sides thereof. A cross wall ll extends from side to side adjacent one end of 60 the kettle l0 and terminates short of the upper edge thereof, defining a spelter reservoir 15.

Radiant tubes l6 extending across the adjacent end of the kettle supply heat directly to the spelter in the reservoir 15.

Sheet-handling mechanism indicated generally at H, largely of conventional construction, is removably disposed in the kettle, resting on the upper edges of the side and end walls thereof. The 70 sheet-handling mechanism includes entering rolls I8, bottom rolls l9 and exit rolls 20, journaled in spaced housings or side frames, together with suitable guides 2| between adjacent pairs of rolls.

Asbestos wipers or the like may be substituted 75 for the exit rolls. A flux box 22 is disposed between the entering rolls l8 and the bottom rolls [9.

In addition to the parts of the sheet-handling mechanism already mentioned, we provide a bailie 23 extending thereacross which is effective to confine a layer of zinc floating on a bath of lead in the kettle III to the area adjacent the exit rolls 20. The lower edge of the baffle 23 and the upper edge of the cross wall l4 extend horizontally to provide abarrier between the lead and zinc to limit the area of contact therebetween.

In placing the apparatus in operating condition, the kettle I0 is filled with molten lead to about the level of the top of the cross wall H. The reservoir [5 is then filled with spelter to about the level indicated by the dotted line 24. Additional lead may then be supplied to the kettle ID to maintain the surface of the lead between the baffle 23 and the cross wall ll substantially at the top of the latter. The fiux box 22 is then filled and the apparatus is ready for use.

Resilient stops 25 are secured to the side walls of the kettle and are provided with tapering V- blocks 26 adapted to cooperate with similar blocks 21 on the mechanism I]. These stops cooperate with the baflle 23 to confine the spelter to the portion of the kettle adjacent the exit rolls 20 while permitting removal of the entire mechanism II when desired. The form of the stops 25 is such as to permit yielding as indicated in dotted lines in Fig. 2, on lateral expansion of the mechanism i1 when heated.

The method of coating which are are enabled to practice by the apparatus described above includes feeding sheets through a flux bath contained in the box 22 and then through a lead bath contained in the kettle III, the surface of which is indicated by the dotted line 28. The lead does not wet the steel sheets but does serve to preheat the latter to prevent excessive chilling of the shallow layer of spelter into which the exit rolls 20 dip. By reason of the independent means for heating the lead bath, it may be maintained at any temperature desired, either that of the spelter bath or at a temperature above or below that value. Similarly, the spelter bath may be maintained at precisely the most effective temperature for coating, more or less independently of the temperature of the lead bath.

0n emerging from the lead and entering the spelter, the sheets acquire a uniform coating of zinc and are then fed upwardly out of the kettle by the exit rolls 20'to the usual conveying means for delivery to further processing apparatus.

By maintaining the level of the lead between the baflie 23 and the cross wall H substantiallly at the top of the latter, any dross forming in the spelter layer and sinking toward the bottom thereof, will naturally flow over the upper edge of the cross wall and collect at the bottom of the settling tank l5 as indicated at 29. To assist this action, I may also employ means such as a paddle wheel 30 driven by a small motor (not shown), for positively withdrawing spelter from the bottom of the layer thereof to facilitate deposit of the dross contained therein at the bottom of the tank 15. The spelter withdrawn by the paddle 30, obviously, is replaced by highly heated spelter from the top of the body of spelter contained in the reservoir IS. The rotation of the exit rolls 2!! may cause sumcient circulation of the spelter to make the wheel 30 unnecessary.

When it is desired to suspend operations, the

spelter and the lead may be bailed out more or lesssimultaneously until their levels are lowered to the upper edge of the wall I 4. Thereafter the two metals may be bailed out individually as desired.

Referring now to Figs. 4 and 5 showing a stripcoating apparatus, a tank 1' is adapted to contain a bath of molten metal, e. g., lead, which does not wet or alloy with steel strip passing therethrough as indicated at H. The surface of the lead bath is indicated at It".

At the entrance end of the tank III a flux box I3 is mounted. Radiant heating tubes H are disposed beneath the box l3 and adjacent the end of the tank l for supplying heat to the contents thereof, respectively. The strip entering the tank passes over a guide roll l5, under rolls i6 over a roll II in the flux box, and thence under a guide roll I8 in the tank Ill. The strip may conveniently be fed into the coating apparatus from a coil or directly from a processing line such as a pickling apparatus.

Immersion heaters IS in the form of heatexchange tubes, are disposed in the tank ill in spaced relation therealong. Each tube has a burner connected to a fuel supply conduit whereby a combustible mixture is supplied to the tubes I9 and, on combustion therein, supplies suflicient heat to the metal in the tank to maintain it in molten condition.

Guide rolls 2| cooperate with the roll l8 and a similar roll 22' adjacent the exit end of the tank to hold the strip below the surface of the lead as it passes through the tank. The rolls 2| are journaled 'on arms 23' secured to shafts 24. The shafts 24' may conveniently be provided with cranks 25' whereby the arms 23 may be swung upwardly as indicated in dotted lines in Fig. 4, to facilitate threading of the strip through the tank l0 when starting operations. It will be understood that after starting; the material is fed through the tank in a continuous strand, successive lengths being joined together end to end in any convenient manner.

A cross wall 26 adjacent the exit end of the tank extends upwardly from the bottom thereof but terminates short of the top. A baille 21' spaced inwardly from the wall 26' extends downwardly from the top of the tank to a point below the top of the cross wall 26. A bath of coating material 28, e. g., molten metal such as zinc, is confined by the cross wall 26' and baffle 21, a portion of the coating bath floating on top of the lead bath. The cross wall and baifie have horizontally extending portions 26a and 27a which further separate the lead from the zinc.

In fact, these portions prevent almost all contact between the lead and zinc. Instead of being fixed, they may be arranged to float on the lead. Exit rolls 29' are journaled between the baflle and cross wall and radiant heat-exchange tubes 30' extend across the top of the tank for supplying heat to the coating bath and heating the strip as it passes upwardly between the exit rolls 29'. Asbestos wipers may be used instead of the exit rolls 29'. A paddle wheel 3| is mounted in the coating bath adjacent the upper edge of the cross wall 26'. The wheel may be driven by any convenient means such as a small motor (not shown). The function of the wheel is to circulate the coating bath as indicated by the arrows to remove dross from the neighborhood of the exit rolls, and by circulating it within the space between the wall 26 and the end of the tank, to cause agglomeration of the separate Particles thereof, whereby they finally co'llect at the bottom of the coating bath as indicated at 32. By this means, we insure that the coating metal adjacent the exit rolls is always clean and free from dross.

A cover 33 extends over the tank ill for the full length thereof and may be provided with an upward extension or tower 34. The cover may be insulated if desired, to limit loss of heat from the molten bath. An inlet 35 for non-oxidizing gas permits a neutral atmosphere to be maintained in the cover, thereby preventing oxidation of the bath at the surface thereof and the resultant formation of skimmings. The neutral atmosphere is normally supplied continuously through the inlet 35 and is discharged at a limited rate through outlets 36 controlled by dampers 31.

The method which we contemplate practicing with the aid of the apparatus of Figs. 4 and 5 will now be briefly outlined. The strip I I, after being made ready for coating by pickling if necessary or some other cleaning process, enters the coating apparatus shown in Fig. 1 and first traverses the flux in the box i3. This flux may be the usual ammonium chloride which is maintained in molten condition by the heat supplied from the tubes M. The strip absorbs a certain amount of heat from the flux and then enters the lead bath. The length of the lead bath' is such that, at a given rate of travel of the strip, it will be heated to the temperature at which it readily alloys with zinc, before it leaves the lead bath. The higher the speed at which it is desired to move the strip through the coating apparatus, therefore, the longer must be the lead bath which serves to pre-heat the strip to the alloying temperature. The heat absorbed by the strip passing through the lead bath is made up by the heat delivered to the bath by the tubes l9 so that the bath remains molten at all times.

When the strip passes from the lead bath into the coating bath 28, it is at such temperature that the zinc spelter immediately forms an alloy with the steel base at the surface thereof and a layer of relatively pure zinc on top of the alloy layer. Because the path of the strip through the bath is relatively short, the opportunity of forming a zinc-iron alloy on the surface of the strip is slight. As a result, only a very thin layer of alloy is formed. We have found that if the strip travels through a path as short as 4" in the coating bath, or even less, a satisfactory coating will result.

As the strip emerges from the coating bath between the exit rolls 29', it is subjected to the heat radiated from the tubes 30'. This causes the coating to remain liquid for some distance above the surface of the bath. Rapid loss of heat from the strip to the atmosphere and resultant solidification of the coating is prevented to some extent by the extension 34 on the cover 33. The atmosphere within the extension is maintained at a relatively high temperature by the heat from the tubes 30' so that the coated strip is subjected to a heat treatment for a deflnite length of time after it leaves the coating bath. As the strip reaches the upper end of the extension 34, it cools to an extent such that the coating is solidified. The strip then passes between pinch rolls 38 and over a guide 39 from which it may be coiled on a reel or fed forward for further processing such as cutting to length or the like. The effect of the heat treatment after application of the coating but before the solidification thereof, is to produce material having a dull or matte surface. If this is not desired,

the extension 34 may be omitted and the strip 5 permitted to cool rapidly on emerging from the coating bath. If this is done, the strip has a high luster and its surface exhibits large interlocking flower or fern-like crystals, instead of the sharply defined spangles usually formed on the sheets as they emerge from a galvanizing bath.

We preferably maintain a slight temperature gradient between opposite ends of the tank ID, the entrance end being maintained at a temperature somewhat higher than that prevailing at the exit end.

We find that the product resulting from the procedure described above, viz., galvanized steel strip, is characterized by the presence of a small amount of lead in the coating, e. g., 1% or thereabouts. This is desirable in that it gives the coating greater durability and greater resistance to corrosion.

Fig. 6 illustrates a modification of the apparatus shown in Figs. 4 and 5, which is particularly adapted for the coating of wire. The principal difference between this apparatus and that just described is that it includes guide rolls 40 in the coating bath whereby the path of the material through the bath is somewhat longer than in the apparatus of Fig. 1. The length of the path may be changed according to the thickness of coating desired, a shorter path being sufllcient for a lighter coating. The remainder of the apparatus is the same as shown in Fig. 1 and the parts thereof are indicated by corresponding reference numerals.

It will be apparent that the apparatus and method of the invention, in addition to limiting 40 the rate of absorption of zinc by the lead, also tends to prevent the accumulation of dross in the neighborhood of the exit rolls where it might be picked up by the base metal as it emerges from the apparatus. The apparatus also makes it pos- 45 sible to produce coated sheet steel unmarred by the streaks or black spots which have been formed after a few days operation when following the conventional lead-zinc coating process.

At the same time, we obtain an important advan- 50 tage over the latter, viz., the .short interval of contact between the steel and zinc which limits the formation of zinc-iron alloy and causes the zinc coating to be tightly adherent to the steel base.

Other advantages, such as the independent con- 55 trol of the temperature of the lead and zinc, have already been mentioned.

These advantages may be emphasized by a few comparative figures. The total amount of dross formed in operations conducted in accordance with the invention amounts to only about 4% of the total zinc used instead of 14% as has been customary heretofore. Instead of the 22% to 24% dross and 16% to 18% skimmings resulting from a conventional wire galvanizing bath, the apparatus and method of my invention involve a total loss of zinc in the form of dross and skimmgrgs, of less than 8%. It will be apparent that the saving in zinc scrap produced represents a substantial reduction in the cost of the product. The quality of the product, furthermore, is superior to that produced by conventional practice. No dross spots are formed on the material since any dross formed in the coating bath is removed from the path of the strip, and the particles 75 thereof agglomerated so that they sink rapidly to the bottom of the coating bath out of the path of the strip. The thickness of the coating may be varied by changing the length of the path of the material through the coating bath. If the material is subjected to the after heat treatment for the purpose of giving it a dull or matte surface, the coating is not subject to crumbling or conversion to powder as is the coating of similar material produced by previous methods. In any case, the coating is highly adherent and withstands severe deformation without fracture or flaking.

While we have illustrated and described but a preferred embodiment and practice of the invention, it will be understood that changes in the apparatus and procedure may be made without departing from the spirit of the invention or the scope of the appended claims. We may find it desirable, for example, to heat the lead from the top by tubes similar to those shown at 30 instead of by the immersion tubes 19'. The apparatus of Figs. 1 and 3, furthermore, may be provided with a cover such as that shown at 33, and non oxidizing gas supplied thereto.

We claim:

1. In a coating apparatus, a container for molten metal having a cross wall adjacent one end forming a section for coating metal, a battle spaced inwardly of said wall, the lower edge of the bafile extending downwardly at least to the upper edge of the wall whereby to confine a layer of the coating metal from said section floating on the surface of another molten metal at about the level 01' the upper edge of said wall, and a horizontally extending plate between said baille and cross wall effective at least partially to separate the coating metal from the other molten metal.

2. Apparatus for coating sheet metal comprising a container for a preheating bath of molten metal which does not wet the sheet metal, a cross wall in said container forming part of a settling tank for molten coating material, a battle cooperating with said wall combining a coating bath floating on said preheating bath and communicating with said tank, means adjacent said coating bath for conducting sheet metal upwardly from said preheating bath through said coating bath, and means for circulating molten coating metal between said coating bath and tank whereby dross forming adjacent said means may settle in said tank.

3. In a coating apparatus, a container for molten metal having a cross wall adjacent one end defining a section for coating metal, a bafile spaced inwardly'of said wall, the lower edge of the bailie extending downwardly at least to the upper edge of the wall whereby to confine a layer of the coating metal from said section floating on the surface of another molten metal at about the level of the upper edge of said wall, a partition between the coating metal and the other: molten metal extending at least partly between said wall and bafile, and means for circulating said coating metal from the space between the ballie and wall to the space beyond the wall thereby causing settlement of dross forming in the coating metal at a point removed from the path of" the material to be coated.

4. In a sheet metal coating apparatus, a molten bathof non-coating metal through which the sheet metal to be coated is passed, a bath of molten coating metal floating on said first-mentioned bath, a transverse baflie confining said coating bath to a relatively small area of said first-mentioned bath, and a horizontal division plate separating said coating bath from said firstmentioned bath at their common surface, except for a space adapted for the passage of the sheet metal therethrough.

-5. Apparatus for coating base metal comprising a container for molten metal, means for conveying base metal to be coated into and through said molten metal, means for confining molten coating metal floating on said first-mentioned molten metal in a relatively small area adjacent the point at which the base metal emerges from the first-mentioned molten metal, a settling tank communicating with said confining means, means for withdrawing from said point, dross forming in said molten coating metal, and delivering it to said tank.

6. Coating apparatus comprising sheet feeding mechanism including spaced side frames adapted to be immersed in a container of molten metal, said mechanism including a baiile extending transversely between said side frames and adapted to confine molten coating metal floating on said first-mentioned molten metal, and stops extending inwardly from the walls of the container into engagement with said side frames, effective to dam said coating metal in the space between the side frames and the container walls.

'7. Coating apparatus as defined by claim 6 and characterized by said stops including interfltting portions on the side frames and container, respectively, one of said portions being yieldable to allow for expansion of the mechanism.

8. In a method of coating sheet metal, the steps including passing the sheet metal through a preheating bath, then through a bath of molten coating metal along a predetermined path, withdrawing coating metal from adjacent the bottom of the second mentioned bath and delivering it to a volume of coating metal deepersthan said second-mentioned bath and spaced from said path, thereby effecting settlement of any dross forming in said coating metal, at a distance from said path.

9. Apparatus for coating base metal comprising a container for molten metal, means for conveying base metal to be coated into and through said molten metal, means for confining a bath of molten coating metal floating on said first-mentioned molten metal in a relatively small area adjacent the point at which the base metal emerges from the first-mentioned molten metal, a settling tank communicating with said confining means, and means for withdrawing coating metal from the bottom of said bath and delivering it to said tank.

-10. In a coating apparatus, a container for molten metal having a cross wall adjacent one end forming a relatively deep section for coating metal, the top of the wall being below the top of the container, a baille spaced inwardly of said wall, the lower edge of the baiile extending downwardly at least to the upper edge of the wall whereby to confine a relatively shallow layer of the coating metal from said section floating on the surface of another molten metal in the container, at about the level of the upper edge of said wall, and means for withdrawing coating metal from the bottom of said layer and delivering it to said deep section.

11. Apparatus for coating sheet metal comprising a container for a preheating bath of molten metal which does not wet said sheet metal, a cross wall in said container providing a reservoir for molten coating metal which does-wet the sheet metal, the top of the wall being below the top of the container, a baflle spaced inwardly of said cross wall to confine a relatively shallow layer oi coating metal floating on said preheating,

bath, means for guiding sheet metal to be coated upwardly through said layer after traversing said bath, means separating saidlayer from said bath except for a slot through which the sheet metal

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