US2824021A

US2824021A - Method of coating metal with molten coating metal

Info

Publication number: US2824021A
Application number: US552630A
Authority: US
Inventors: Nelson E Cook; Samuel L Norteman
Original assignee: Wheeling Steel Corp
Current assignee: Wheeling Steel Corp
Priority date: 1955-12-12
Filing date: 1955-12-12
Publication date: 1958-02-18
Anticipated expiration: 1975-02-18

Description

Feb. 18, 1958 N. E. cooK ErAL 2,824,021

METHOD oF coATING METAL WITH MoLTEN .coATING METAL original -Filed Feb. 24, 1954 2 sheets-sneer 1 sa] '2s 25 INVENTORS NELSON E COOK Bs SAMUEL L NORTEMAN Feb. 18, 1958 N. E. cooK Erm. 2,824,021

METHOD oF coATING METAL WITH MOETEN COATTNG METAL Original Filed. Feb.` 24, 1954 2 Sheets-Sheet 2 l FIGA; I Fla-5, |8 2 2 22 22 f s, Mw M Mfr MMO x M MMM INVENToRs NELSON E COOK a SAMUEL L NQRTEMAN METHOD F COATING METAL WITH MOLTEN COATING METAL Nelson E. Cook and Samuel L. Norteman, Wheeling, W. Va., assignors to Wheeling Steel Corporation, Wheeling, W. Va., a corporation of Delaware Continuation of applications Serial Nos. 412,265 and 412,266, February 24, 1954. This application December 12, 1955, Serial No. 552,630

2 Claims. (Cl. 117-51) This invention relates to a method of and apparatus for coating metal wtih molten coating metal. This application is a continuation of our copending applications Serial Nos. 412,265 and 412,266, filed February 24, 17954.

While the invention has other aspects and may be otherwise embodied its utility is at present realized primarily in the tight coat hot dip galvanizing of metal and for purposes of explanation and illustration the invention will be described in connection with the tight coat hot dip galvanizing of metal and specifically the continuous tight coat hot dip galvanizing of steel strip. The invention is applicable to the hot dip galvanizing of ware by hand or machine and to the hot dip galvanizing of wire. We use the term strip, unless the context clearly requires otherwise, as a word of broad definition and not of limitation and to comprehend metal in elongated liexible form including wire.

Tight coat hot dip galvanizing involves the admixture with zinc in the molten coating bath of another metal or metals whose function is to inhibit the formation of brittle zinciron alloy, which inhibition is a prerequisite to the production of a tightly adherent coating. The metal which may be added to the zinc may be aluminum, lead, antimony, cadmium, tin, etc., either singly or in various combinations. The metal most commonly used is aluminum and it will be considered as an example of the various metals and combinations of metals which may be added to zinc to form a tight coat hot dip galvanizing bath. The added metal is in very small percentage in relation to the zinc, normally constituting not over one or two percent by weight of the total content of the bath.

Some of the added metals of which aluminum is an example tend to react with linx, especially linx containing chlorine compounds, which is the type of linx preferred, so it is not practicable to float the linx on the surface of the molten coating bath. The metal is preliuxed, i. e., the linx is applied to the metal before it enters the receptacle or pot containing the coating bath. As soon as the tiuxed metal enters the bath the linx reacts with the added metal, here considered by way of example as aluminum, with the result that there is a tendency for the coating bath in the vicinity of the point of entry thereinto of the metal to be coated to become relatively lean in aluminum. That is because of the reaction between the aluminum and the chlorine in the linx. Indeed, it has` been found that that reaction is so rapid and violent that in some cases as much as seventy to eighty percent of the aluminum in the coating bath is consumed by reaction with the chlorine of the liux.

Since effective tight coat galvanizing is dependent upon maintaining thorough and substantially uniform admixture of the zinc and aluminum in the coating bath it will be Iappreciated that the tendency of the bath to become lean in aluminum in the vicinity of the point of entry thereinto of the metal to be coated is serious. If that rates arent OI ICC tendency were not inhibited there would not be a sutiicient action of the aluminum during the coating to prevent the formation of brittle iron-zinc alloy snliicient to result in :a coating which would flake and peel when the galvanized metal is subjected to drawing operations.

We inhibit the tendency for the molten coating metal in the vicinity of the point of entry thereinto of the liuxed metal to be coated to become relatively lean in aluminum by stirring the metal of the coating bath by electric induction heating. We discovered that electric induction heating has the effect of maintaining a uniform bath and thus promoting tight coat galvanizing as explained in our Patent No. 2,647,305. We have found that the stirring brought about by the induction heating suiciently inhibits the tendency of the coating bath to become lean in aluminum in the vicinity of the point of entry into the bath of the fluxed metal to be coated that a high quality product is produced. The induction heating maintains the coating bath in rapid and thorough circulation so that virtually as rapidly as laluminum is removed by reaction with thechlorine of the flux the aluminum is replaced by other aluminum in the bath through circulation. Of course, there is a gradual loss of aluminum in the total bath but that loss iscompensated for by aluminum additions. The stirring effected by the induction heating also brings about rapid dissemination and distribution throughout the bath of the aluminum additions.

When strip such as steel strip is being continuously tight coat hot dipped, galvanized guide means are necessarily provided in the galvanizing wpot which are at least partially immersed in the coating metal to guide the strip in its passagethrough the molten coating metal. It is important that the flux on each increment of the length of the strip Ibe virtually entirely consumed before that increment of the length of the strip engages the guide means in the bath as otherwise the quality of the product Will be adversely affected. To accomplish that result the linx which is applied to the strip before galvanizing is dried and also heated before the strip enters the bath, the heating preferably being to such a temperature as to accelerate the reaction between the linx and the aluminum in the bath so that the reaction is substantially complete before the strip is engaged by the'guide means in the coating pot.

Other details, objects and advantages of the invention will become apparent as the following description of a present preferred embodiment thereof and a present preferred method of practicing the same proceeds.

In the accompanying drawings we have shown a present preferred embodiment of the invention and have illustrated a present preferred method of practicing the same` in which:

Figure 1 is a diagrammatic elevational view of apparatus for liuxing and coating metal strip;

Figure 2 is a view to enlarged scale partly in elevation and partly in axial cross section of one of the rubber surfaced rolls for controlling the coating of flux on the strip;

Figure 3 is a view to enlarged scale partly in elevation and partly in axial cross section of one of the brushes for controlling the coating of linx on the strip;

Figure 4 is a fragmentary axial crosssectional view to enlarged scale of the roll shown in Figure 2; and

Figure 5 is a fragmentary axial cross-sectional view to enlarged scale of the brush shown in Figure 3.

Referring now more particularly to the drawings, 1 designates by way of example steel strip to be galvanized. in Figure l the strip is shown as passing in the direction of the arrows lirst about a guide roll 2 and then about guide rolls d, lil, 13 and 28 as shown. The power for advancing the strip may be applied by a reel (not shown) which coils up the coated strip and which may, if desired, be supplemented by other driving rolls in the line.

The guide rolls 4 are mounted with their lower portions immersed in a bath of flux in water solution in a receptacle 3. The guide rolls 4 are shown as having their axes slightly above the surface of the flux solution. The ilux solution will, for purposes of example, be deemed to be a water solution of 45% ammonium chloride and 55% Zinc chloride. The strip 1 moves vertically upwardly from the iluX bath as shown. There lis a tendency in the flux bath for more flux to adhere to the surface of the strip disposed away from the guide rolls i than to the surface of the strip disposed against the guide rolls, and to compensate for that tendency ux solution is introduced through a pipe 5 and flowed into the crotch 6 between the right-hand guide roll 4 and the strip as shown in Figure l. This insures the application of a relatively uniform coating of flux to the respective faces of the strip. The bath of flux is replenished to compensate for the flux removed on the strip. The flux solution may have a specic gravity of -20o Baume and a temperature of ISO-200 F. The ilux solution may, if desired, be continuously recirculated and filtered.

As the strip moves upwardly from the crotch 6 it carries on both faces a relatively heavy layer of ux solution. The strip passes between rubber surfaced metering rolls 7 and levelling brushes 8 which may, for example, be nylon bristle brushes. The rolls and brushes are for the purpose of removing part of the flux solution and insuring that the unremoved ilux solution is in a smooth uniform coating on the strip. The rolls and brushes may all be used at the same time or only selected ones thereof may be used, depending on conditions. Those of the metering rolls 7 which are in use at any given time preferably turn in the direction in which the strip tends by friction to rotate them and at the same surface speed as the strip. They are preferably rotated by frictional engagement with the strip, although they may be independently driven. The metering rolls 7 are preferably grooved. The portions of the metering rolls '7 which engage the strip squeeze off the flux solution and the grooves meter the amount of flux solution which can pass through, later to be smoothed out or levelled by the levelling brushes ti. The levelling brushes SS may be rotated or not and if they are rotated they may be rotated in either direction at selected speeds. Normally the levelling brushes 8 will be rotated slowly in the direction opposite the direction in which the strip tends by friction to rotate them.

Figure 2 shows one of the rubber surfaced metering rolls 7 and Figure 4 is a fragmentary View of the roll to enlarged scale. The left-hand half of the roll viewing Figure 2 is shown in elevation and the rightfhand half in cross section. The roll comprises a cylindrical body i6 having integral trunnion-like ends l5 on which the roll is mounted for rotation. The roll body 16 has disposed thereabout a hollow rubber cylinder il? having therein spiral grooves 1S. Preferably opposed rolls 7 are alike with respect to the spacing, pitch and cross section of the spiral groove but the upper set of rolls 7 may diifer from the lower set of rolls 7 by having the grooves thereof of different pitch and/or different spacing and/ or different cross section. The rolls of each pair engage the strip at opposite faces. They reduce in controlled manner the thickness of the coating of flux solution on the strip. For a particular strip speed only one of the sets of rolls 7 may be used, for a different strip speed only the other of the sets of rolls 7 may be used and for a still different strip speed both sets of rolls 7 may be used simultaneously. Under certain conditions the brushes may alone be adequate so that neither set of rolls 7 may be used.

Figure 3 shows one of the levelling brushes 8. A portion of the brush is shown fragmentarily to enlarged scale in Figure 5. Referring to Figure 3, a shaft 19 carries the brush. The left-hand half of the brush is shown in elevation and the right-hand half in cross section. The brush comprises a sleeve 2t) carrying bristles 2l which for the purpose of illustration may be deemed to be of nylon. The bristles may be attached in separate tufts Z2 as shown in Figure 5. However, the

ristle ends lie substantially in a right cylindrical surface. The brushes 3 may be used either alone or in conjunction with either or both of the sets of rolls 7, or under certain conditions the brushes may be rendered inoperative and one or both of the sets of rolls alone employed. When the brushes are operative they act against the opposite faces of the strip and are preferably rotated slowly in the direction opposite the direction in which the strip tends by friction to rotate them. They tend to smooth and distribute the coating of ux solution on the strip to produce a very uniform even coating. By selection of desired rolls 7 and/or brushes 3 and rotation of the brushes at desired speed the thickness of the coating on the strip may be closely controlled.

As the strip carrying a controlled coating of liquid ilux solution passes upwardly from the brushes 8 it enters a heating chamber 9 in which the strip is heated to evaporate the water in the flux solution. The heat may be supplied by any appropriate means, such as electric heating elements, heated air, products of combustion, etc. The strip is free from engagement by any portion of the apparatus after it leaves the brushes 8 until it reaches the left-hand guide roll 10. As a consequence the flux is dried on the strip uniformly and smoothly. The uniformity and smoothness of the coating of ux on the strip might be impaired if the strip were engaged by any portion of the apparatus during the drying operation and before that operation had been substantially completed.

The strip reaching the left-hand guide roll 10 therefore has on both faces a substantially dry uniform coating of flux at a temperature of the order of 250 F. The strip passes over the guide rolls l@ and thence downwardly toward the galvanizing pot which is designated generally by reference numeral l2 and which contains a molten coating bath 14 which may be deemed to be zinc with a small percentage, say, less than one percent., of aluminum admixed therewith. In its downward passage the strip passes through a further heating chamber l1 wherein its temperature is raised to a temperature approaching the maximum temperature of stability of the dry flux coating on the strip. That temperature in the case of ilux containing 45% of ammonium chloride and 55% of zinc chloride is 450-500 F. The heat in the heating chamber 11 may be applied by any of the methods mentioned in connection with applying heat in the heating chamber 9.

Thus as the strip enters the bath of molten spelter in the galvanizing pot i2 its temperature relatively closely approaches the temperature of the molten spelter with the result that the dry flux coating on the strip reacts with the spelter with relatively great rapidity. The reaction is so rapid that it is substantially complete by the time the strip reaches the left-hand guide roll 13 in the galvaniziug pot, viewing Figure l. The coated strip emerges from the galvanizing pot 12 between the rolls 27 which may be the conventional exit rolls of the galvanizing rig.

As explained above, the reaction between the aluminum in the bath and the chlorine in the fiux consumes a substantial if not major portion of the aluminum in the bath, that consumption occurring in the vicinity of the point Where the fluxed strip enters the bath. The tendency of such consumption of the aluminum is to make the bath in the vicinity of the point of entry thereinto of the strip relatively Vlean in aluminum. As above explained, that tendency is counteracted by the rapid and thorough circulation of the metal of the bath brought about by electric induction heating.

The galvanizing pot 12 comprises an outer steel shell 29, a lining 30 of insulating material within the shell 29 and a refractory inner lining 3l for containing the molten coating bath. The pot has a lateral extension designated generally by reference numeral 32 which has a refractory wall 33 communicating with the refractory lining 31 of the pot proper which forms a passage 34 which extends laterally and downwardly from a side of the pot. The passage 34 communicates with the interior of the pot at the upper portion thereof as shown in Figure l and extends outwardly and downwardly. The passage 34 may be positioned at various elevations in the pot depending upon considerations of manufacture and use. The cross section of the passage 34 preferably decreases from relatively great cross section where it enters the pot to relatively small cross section at its outer portion. The passage 34 communicates with three downwardly and outwardly directed ducts 23 which at their outer extremities enter a cross duct 24. Disposed in a closed loop about the central one of the three ducts 23 is a magnetic core 25 having formed on each of the legs thereof extending between the central duct 23 and an outside duct 23 a coil 26 to which electric current is conducted. The current conducted to the coils 26 may, for example, be 440 volt, single phase, 60 cycle, alternating current. There may be provided a pack of static capacitors for power factor correction, contactors for switching from high to low voltage for temperature control, a circuit breaker and auxiliary equipment for automatic control of power input to maintain a desired temperature in the body of the molten coating bath in the pot.

A low voltage is induced across the ducts 23. The apparatus is analogous to a stepdown transformer having a short circuited secondary which is formed by molten metal filling the

ducts

23 and 24. Since the

ducts

23 and 24 form a closed circuit a current will flow in them. The current generates heat in the metal itself. That heat may be the sole heat for maintaining the bath molten or it may be supplemented by heat from another source.

The heat generated in the metal causes a continuous and rapid circulation of the molten coating bath across the pot as shown by the arrows in Figure 1. That circulation maintains the molten coating bath of relatively uniform composition through out the portion of the pot through which the strip passes in being galvanized. It also, as above explained, counteracts the tendency of the molten coating metal in the vicinity of the point of entry thereinto of the strip to become relatively lean in aluminum, such counteracting being due to the rapid and thorough circulation of the molten coating metal elfected by the electric induction heating means.

While we have shown and described a present preferred embodiment of the invention and have illustrated a presvanized coating, which molten metals should be maintained thoroughly and uniformly admixed for optimum results, compri-sing stirring said molten coating bath by electric induction heating to maintain the zinc and other metal thoroughly admixed, applying to a metal strip tlux which reacts with the metal other than zinc in the coating bath at the temperature of the molten coating metal, advancing the fluxed strip in the direction of its length into and through the molten coating metal at a speed such as to create a tendency for the molten coating metal in the vicinity of the point of entry thereinto of the strip to become relatively lean in the metal which reacts with the flux, the stirring of the molten coating metal by electric induction heating being at a rate counteracting said tendency of the molten coating metal in the vicinity of the point of entry thereinto of the strip to become relatively lean in the metal which reacts with the flux due to rapid and thorough circulation of the molten coating metal.

2. A method of tight coat hot dip galvanizing metal strip by use of a molten coating bath comprising molten zinc and a relatively small quantity of molten aluminum admixed with the molten zinc to provide metal strip having a tight galvanized coating, which molten metals should be maintained thoroughly and uniformly admixed for optimum results, comprising stirring said molten coating bath by electric induction heating to maintain the zinc and aluminum thoroughly admixed, applying to a metal strip ux which reacts with the aluminum :in the coating bath at the temperature of the molten aluminum, advancing the iluxed strip in the direction of its length into and through the molten coating metal at a speed such as to create a tendency for the molten coating metal in the vicinity of the point of entry thereinto of the strip to become relatively lean in aluminum, the stirring of the molten coating metal by electric induction heating being a-t a rate counteracting said tendency of the molten coating metal in the vicinity of the point of entry hereinto of the strip to become relatively lean in aluminum due to rapid and thorough circulation of the molten coating metal.

References Cited in the le of this patent UNITED STATES PATENTS Attestng Officer UNITED STATES PATENT OFFICE CER'IFICTE @F CRRECTION Patent No. 2,824,021 February l8, 1958 Nelson Cook et alo y It is hereby certified. that error appears in the printed specification of the above numb ered patent requiring correction and that 'the said Letters Patent should reed es oorreoted belowu Column 2, line 3l, for "di pped, 'Y read dip w; column 5, line 4'?, for "through out" reed n throughout n; oolwnn 6, line 46, for "hereinto" read m thereinto Signed and sealed this 22nd day of April 1958,

(SEAL) Attest:

KARL H., AXLINE ROBERT C. WATSON Conmissioner of Patente

Claims

1. A METHOD OF TIGHT COAT HOT DIP GALVANIZING METAL STRIP BY USE OF A MOLTEN COATING BATH COMPRISING MOLTEN ZINC AND AT LEAST ONE OTHER MOLTEN METAL ADMIXED WITH THE MOLTEN ZONC TO PROVIDE METAL STRIP HAVING A TIGHT GALVANIZED COATING, WHICH MOLTEN METALS SHOULD BE MAINTAINED THOROUGHLY AND UNIFORMLY ADMIXED FOR OPTIMUM RESULTS, COMPRISING STIRRING SAID MOLTEN COATING BATH BY ELECTRIC INDUCTION HEATING TO MAINTAIN THE ZINC AND OTHER METAL THOROUGHLY ADMIXED, APPLYING TO A METAL STRIP FLUX WHICH REACTS WITH THE METAL OTHER THAN ZINC IN THE COATING BATH AT THE TEMPERATURE OF THE MOLTEN COATING METAL, ADVANCING THE FLUXED STRIP IN THE DIRECTION OF ITS LENGTH INTO AND THROUGH THE MOLTEN COATING METAL AT A SPEED SUCH AS TO CREATE A TENDENCY FOR THE MOLTEN COATING METAL IN THE VICINITY OF THE POINT OF ENTRY THEREINTO OF THE STRIP TO BECOME RELATIVELY LEAN IN THE METAL WHICH REACTS WITH THE FLUX, THE STIRRING OF THE MOLTEN COATING METAL BY ELECTRIC INDUCTION HEATING BEING AT A RATE COUNTERACTING SAID TENDENCY OF THE MOLTEN COATING METAL IN THE VICINITY OF THE POINT OF ENTRY THEREINTO OF THE STRIP TO BECOME RELATIVELY LEAN IN THE METAL WHICH REACTS WITH THE FLUX DUE TO RAPID AND THOROUGH CIRCULATION OF THE MOLTEN COATING METAL.