US2566468A - Method of treating electrolytic coatings - Google Patents

Description

Sept. 4, 1951 E.-S. TAYEE RSON I METHOD OF TREATING ELECTROLYTIC CUATINGS Original Filed im 22, 1942 2 Sheets-Sheet 1 za fiam "2 N W mi MM m .7 z w... fi 4 N i: 4. 8

n J j y Sept. 4, 1951 E. s. TAYLERSON 2,556,458

METHOD OF TREATING ELECTROLYTIC COATINGS Original Filed, June 22, 1942 2 Sheets-Sheet 2 Patented Sept. 4, 1951 METHOD OF TREATING ELECTROLYTIC COATINGS Ewart S. 'Taylerson, Pittsburgh, Pa., assignor to United States Steel Company, a corporation oi New Jersey Original application June 22, 1942, Serial No. 447,966. Divided and this application March '7, 1947, Serial No. 733,235

6 Claims.

This invention relates to coatings on ferrous metals especially toferrous metals plated with a coating having a low melting point such as tin coatings, and particularly to an improved method .of treating electrolytic tin coatings on strip steel so as to obtain a maximum surface luster and/or a bonding of the tin coating with the base metal of the strip. The present invention is a division of my copending application, Serial No. 447,966, filed June 22, 1942, which has become abandoned.

It is well known that in the hot-dipped method of coating iron or steelthere is provided a bright surface finish and an alloy bond between the coating and the base metal. However, such a method of tinning ferrous metal is objectionable for the reason that the coating varies widely from one point to another upon a hot-dipped sheet. Furthermore, an excessive amount of tin is used in the hot-dipped method, thereby making this process relatively expensive. In an attempt to overcome these objectionable 1 features, it has been suggested that the coating be deposited on the ferrousv metal electrolytically.

Such method provides a substantially uniform tin coating on the sheet or strip, but any alloying of the. tin coating with the base metal is not obtained by this method. Also, the tin-coated surface produced bythe electrolytic method is usually dull and white and there is not provided a brilliant metallic finish characteristic of the hot-dipped method. Heretofore, it has been suggested to heat the tin. coating so as to melt the .same by means of hot oil, gas or electrical energy. Of these methods the use of hotoil has been the most satisfactory as it provides a uniform heating thev strip material. The hot oil method, however, imposes a number of limitations, the first of whichis, it does not permit ready control of heating and cooling to compensate for various speeds oftravel of the strip material; secondly, its use entails expensive and elaborate auxiliary equipment for the removal of the oil from the surface in the preparation of the strip product; thirdly, owing to the viscosity of the oil a maxi- .mum operating speed limitation is imposed in order .to keep the oil in the receptacle. The main disadvantage of the gas method is that it is difficult to obtain and maintain uniform heatingconjditions. Also, rapid changes of strip speed are impossible due to the heat capacity of the furnace and heaters, Also, it is difficult to obtain a .uniform heating with gas.

The utilization of electrical energy as a heating medium, i. e. by utilizing the electric resistance ofthe strip to the passage of electric current therethrough, has numerous advantages from 2 the standpoint of controlling the melting of the tin coating; however, the employment of such electrical method of heating results in a nonuniform heating across the Width of the strip. This is due to the physical shape of the strip material, Non-uniform heating in turn results in a non-uniform appearance of the tin coating after quenching. I have found, however, that the limitations of the electric resistance heating method can be overcome and its advantages utilized if a, substantially isothermal shield is imposed between the strip and the surrounding atmosphere, so as to prevent heat losses from the strip. "'"If such an isothermal shield is employed and the coating on the trip simultaneously melted by the passage of electric current therethrough, I have found that a uniform appearing tin-coated product having a high luster will be obtained and that the coating is mechanically bonded to the base metal.

Accordingly, it is the general object of the present invention to provide an improved method of treating electrolytically coated ferrous metals having a low melting point coating there'- on by which there is provided a coated surface having a high luster which is uniform in appearance.

It is another object of the invention to provide an improved method of treating electrolytically coated'strip steel having a low melting point coating thereon which is simple and inexpensive in its use and, at the same time, efiicient and effective, and which can be conveniently incorporated with existing electrolytic coating equipment.

It is a more specific object of this invention to provide an improved method of treating electroparticularly pointed out in the appended claims.

In the accompanying drawings there is shown, for the purpose of illustration, an embodiment which my invention may assume in practice.

In these drawings:

Figure 1 is a vertical section through apparatus for practicing my invention; and

Figure 2 is a wiring diagram of the apparatus as shown in Figure 1.

Referring more particularly to the drawings,

which is non-corrosive, such as copper or nickel.

It has also been found that stainless steel is practical for such use. Along the outer sides of the side walls 4 of each of the passageways .13 there are arranged electric grid heating elements. In the present instance elements which are conventional space heaters. On the outer sides of the passageways 3 and the side walls 4, there is disposed preferably insulation 6 so as to limit heat losses from the space heaters ,5.

At the lower end of the chamber or body member 2 and to one side thereof there is arranged a receptacle .or hollow body member I which cornmunicates with and is connected to the passageway 3 .thereabove preferably by means of flexible sealing gasket 8. At the lower end of the body member on the opposite side thereof there is arranged a similar receptacle crhollow body mem- .ber .9 in which .a quenching liquid is adapted to be disposed. The receptacle 9 communicates with the passageway 3 thereabove and is connected thereto preferably bymeans of a vertically arranged conduit member which is preferably thermally and electricall insulated from the side walls 4 at IS. The lower end of the vertically arranged member H3 is so disposed that the lower end thereof is adapted to extend down into the quenching liquidin the receptacle 9 so as to we vent air from entering the passageway 3 there- .above.

Within each of the receptacles l and 9 thereis rotatably arranged .a metallic conductor .roll l2 and there are provided electric brushes which are connected preferably to the secondary of a transformer for supplying current thereto. .At .the upper end of the chamber body member 2 in a hollow portion thereof with which both pas- .sageways 3 communicate, there is disposed a rotatable deflector ro1l l3 whichmay be .a conductor roll but which is preferably an idler roll of low heat capacity which is preferably thermally and electricallyinsulated from the other parts of -.the body member -2 in any suitable manner. The

11011 I3 .isarranged so that the peripher there a of is in tangential alignment with each of the passageways 3. Above the roll l3 there-is :dis-

these grid heating elements are shown as a plurality of spaced apart heating below the melting point of tin, for a purpose here inafter to be described.

At the sides of cache: the 1conductor'rolls I2, as shown in thewiring' diagram, there are arranged electric brushes 22 which establish an electric contact between the rolls I2 and the sec- 'ondary of a transformer 23. One side of the priposed a cover I6 for closing the top of the chamher and passageways3.

In the wiringdiagram shown in Figure :2 of the drawings, one side of the strip heaters 5 is connected to one side of a power line by means of wires I1 and the other side of the strip heaters .is connected to the other side of the power line by means of the wires l8 through a conventional type standard-temperature control unit l9. There is arranged preferably at the lower end of the passageway 3 at the meltingzone thereon adja cent the quench tank 9, a thermocouple 2llwhich is connected to the temperature control l9 by means of the wires 21. It isthe purpose-of the thermocouple 20 in conj unctionwith the controller Is to automatically control the temperature of the walls of thepassageway .3 so as to maintain the temperature of the walls at a temperature mary of the transformer 23 is connected to a suitable source of power by means of a wire 24 and the other side of the primary of the transformer is connected to the .power line by means of a wire 25 in which there is arranged preferably a saturated core reactor or suitable induction regulator .26 for regulating the current passing to the transformer. The current passes from the conductor rolls 12 through the strip S which is disposed over and around the conductor rolls l2 and the insulated roll 13 at all times during the continuous passage of the strip through the apparatus, where- "by heat is generated in the strip S due to theresistance of the strip to the passage of electric current therethrough. J I jlit will be unders'tood that the chamber body member 2 together with the receptacles '1 and 9 are arranged in the electrolytic tin coating line :at any suitable point. therein after the plating operation but it is positioned preferably at {a point immediately after the rinsing, anddryi-ng' equipment. The improved apparatus is used in the fcllow 'ing manner for carrying out-the method of the present invention. The ferrous metal or steel tin-coated strip S as itleaves' the drying .operation in the electrolytic tin coating'line passes shown) downwardly into and through .a vertically arranged conduit member l4 extending up wardly from the top of the receptacle 1. .It is important that the surface of the tin-coated strip be in a dry condition as .it enters there; ceptacle *1 of "the present invention. The strip then passes .into'the receptacle 1 and around the conductor roll l2 therein. From the r'ecep; tacle l-the strip passe into the passageway 3 thereaboye over and'around the deflector roll 1:3 in the top of the body.member and down .into .and through the passageway 3 on the opposite 'sidecf the body member into the conduit .memher It! and into thereceptacle 9. The strip then passes into the quenching liquid in the receptacle 8 and around the conductor roll 12 there in and upwardly out of the receptacle to the next operation tobe performed on the strip.

.As thestrip passes through the apparatus, as ha been described, the portion thereof extending between the conductor roll [2 in therecejtacle v1 up through the passageways "3 and over the deflector roll 13 andthe conductor roll 12 positioned .in the receptacle '3, acts as'an electric resistor .and is "heated by thepassage of electric current therethrough and between the; conductor rolls [2. The conductor rolls [2 are connected to .a source of power, .for. m fth secondary or the transiormenes show n h wiring diagram of Figure 2. It will be understood that thestripis gradually heated at it passes .i'rom the conductor roll 12 andthe recept cle I. over the roll. .13 to the conductor Troll 12 in the receptacle .9. ,It will be seen that strip is heated from theinside rather than .from the outside .due to the steel base .of strip itself acting -as the heatingelement. .The current supplied to the strip for heating theLsame is controlled so that strip .and tin coating thereon will not reach a temperature at which the tin.

coating becomes molten until the strip is" about to leave the passageway 3. In other words. the

speed of the strip and the current supplied theretoare balanced so that the tin coating will become molten at a point immediately'before the strip passes into the quenching liquid in the receptacle 9 and not before; It will be understood that inasmuch as heat losses from the strip are eliminated by the efl'ect of the heated side walls 4 of the passageways 3, the value of the current and voltage required to maintain the above point of melting with respect to the the tin coating on the strip but are provided merely. for the purpose of maintaining the, walls 4 of the passageways 3 at a uniform temperature slightly below, i. e., approaching the melting point of the tin, namely to a temperature of about 410 F., themelting'point of tin being about 450 F'., so as to prevent thermal losses while the strip is being heated by the passage of electric current therethrough in a manner hereinbefore described. In other words, the walls 4 of the passageway 3 act a an isothermal shield so as to prevent loss of heat by radiation. It willbe understood that the cross sectional area of passageways 3 is relatively small so as to minimize heat losses due to convection. It will be seen that by heating the walls 4 of the passageways 3 ata uniform temperature slightly higher or at least as high as the temperature of the strip radiation, losses from the strip are effectively eliminated. It hasbeen found that by electrically heating the strip and at the,- same time heating the walls of an enclosed chamber to a. temperature below the melting point of the tin coating there is provided a uniform heating of the strip, thereby eliminating the objectionable features obtained by merely heating the strip by electric resistance alone."

It is important that the heating elements 5 maintain the walls 4 at a temperature in the vicinity of the melting point of tin, preferably slightly below the melting point for the purpose of controlling the heating of the tin coating. In other words, the thermal capacity of the walls is such that if they are maintained too hot control of melting is lost during a change in seed of the strip. It will be understood that the maintenance of the temperature of the walls at or below the melting point of tin is particularly important if the electrolytic line is stopped for any reason. For example, if the walls 4 were maintained above the melting point of tin damage to the coating and the top deflector roll would occur during the periods when the line was stopped. Such damage would result from the heating of the strip by the walls 4. spotty appearance would result on the strip. Also, tin would be transferred to thetop roll and would result in mechanical damage to the coated strip and the roll which, of course. is objectionable. On the other hand, by heating the strip by electric resistance and merely heating the walls around the strip to a temperature sumcient to prevent heat losses from the .strip as it is being heated by resistance a control of is shut down for any reason.

the tin coating thereon can be conveniently and accurately maintained if the electrolytic tin line Thi is one of the mostimportant aspects of the present'invention. By heating the strip by electric resistance, it has been'found that the heating of the tin coating on the strip to' a molten state is stopped almost instantaneously when the current is shut oil. Consequently. it will be een that the objectionable features obtained by heating the tin coating by an exterior heating means have been entirely eliminated.

As a result of my invention, it will be seen that there is provided an improved method of heating the tin coating on strip, whereby thebase metal together with the coating metal is uniformly heated. After the strip together with the tin coating thereon has been heated to such a uniform temperature and immediately quenched it has been found that there is provided a smooth and even tin-coated surface having a brilliant finish and a high luster so as to provide a tincoated strip having an enhanced appearance and a finish far superior to those finishes obtained by heating and quenching means heretofore suggested and used.- g

While I have shown and described one specific embodiment of my invention, it will be understood that this embodiment is merely for the purpose of illustration and description and that various other forms may be devised within the scope of my invention as defined in the appended claims.

I claim:

1. The method of treating steel strip having a coating of tin electrodeposited thereon in which the strip while in motion as a strand is heated to the melting point of the coating by passing a controlled electric current therethrough, characterized by passing said strip during said heat ing 'by said current through a shield of a heat conducting material around said strip, imparting heat to'the outer surfaces of said shield, controlling the amount of heat imparted to said shield independently of said controlled electric current through said strip to maintain said shield at a. temperature approaching but below the melting point of tin whereby a uniform'melting of the tin coating across the width of the strip is achieved and control of said melting remains in the aforementioned controlled current, and quenching the strip immediately after the tin coatm becomes molten.

2. The method of treating steel strip having a coating of tin electrodeposited thereon in which the strip while in motion as a strand is heated to the melting point of the coating by passing a controlled electric current therethrough, characterized by passing said strip during said heating by said current through a shield of a heat conducting material around said strip, and imparting heat to the outer surfaces or said shield at a rate which will maintain the shield at a.

temperature sufiicient to prevent heat transfer from the strip to the shield but below that which would cause the shield to impart heat to the strip whereby non-uniform heat loss from the strip is prevented and a uniform melting of the tin coating across the width of the strip is achieved. and quenching the strip immediately after the tin coating becomes molten.

3. The method of treating steel strip having a coating of tin electrodeposited thereon in which the strip while in motion as a strand is heated ,bu'thelow the-melting point of tin-whereby nonuniform heat lossfrom the strip -zis prevented and a-uniform meltin of the tin coatin a ross the width of the strip is achieved, and uenching the strip immediately after-the becomes mol en.

4;. The method of treating steel strip having as-coating of tin-electrodeposited thereon in which the a-trip while in motion aSastrand-is heated vto th melting p intoi-the coating .hy passinga controlled electric current therethro gh. character ized hy passing said strip, during said heating by ,said passage of said current therethrou h,

through a metallic shield around said strip, imiparting heat to theouter surfaces of said shield atva rate which will maintain the shield at a item: perature of about 410 F., and controlling the rate of "heating ,of the shield independently -of the control of said electric current-passed through the strip to prevent the shield from exceeding said temperature wheneby a :uniform melting of the tin c atin ac oss the width-of he strip achieved but-the con ro of s idcmelting remains in the aforementioned controlled current, and quenching the strip immedia l after the .coat ing becomes molten.

5. The method of treating steel strip ,having a coating of tin ele trodcposi ed thereon in which th s rip is heated to the melting point of t n aby'the p sa e of a elcctric curren therethmugh while the st ip isiin motion as astrand, said elec- "trio current being regulated in accordance with peed of travel of thestrip. characterize by passme aid, str p ..durin said heating throu h a metallic shieldaroundsaidstrin, imparting heat o the. outer suria e .oflsaid shi ldiata ate whi h ,will maintain the shield at a temperature of about 410 F. and controlling the rate .of ,heating' of the shield independently of the speed of travel or said strip to prevent the shield from exceed- .inz said temperature whereby Ea uniiorm meltin of the tin coating across the width of. he aim? is achieved but the control of team m lting Irrrnains the .aioreinentioned controlled ;current. and qu nching the strip immediatel after he coating becomes-molten. i

6. -'I:-he met od of trea ing teel hav n a coatingoftin electrodeposited thereon: in which the strip is heated to the melting point of :tin b lthepassag ran electric current .therethroiteh while the strip -isin motion ,as :a strand, said electric current being regulated in accordance with speed of travelof the strip. characterized by P ssing said strip during saidzheat ng throug a metallic shield around said strip, imparting heat to the outer surfaces of said shield. controlling the amount of heat imparted to said ield in penden l of the speed of rav l mi said-strip t maintain said shield at a temp ature approaching but below the meltin Point of tin lwherebya uniform melting of the tin coating across the widthiof the strip isachievedbut control -'of ,said melting remains in the atonementionedreigulation of current through the strip in accordance with the speedof travel-of thestrih, and quenching the strip immediately after the coatingbecomesmolten. Y

EWART s. .TAYLERSON.'

REFERENCES CITE-D The following references :are of record in the file of this patent:

UNITED STATES PATENTS France vv "Jan." .6, 1937

Claims (1)

1. THE METHOD OF TREATING STEEL STRIP HAVING A COATING OF TIN ELECTRODEPOSITED THEREON IN WHICH THE STRIP WHILE IN MOTION AS A STRAND IS HEATED TO THE MELTING POINT OF THE COATING BY PASSING A CONTROLLED ELECTRIC CURRENT THERETHROUGH, CHARACTERIZED BY PASSING SAID STRIP DURING SAID HEATING BY SAID CURRENT THROUGH A SHIELD OF A HEAT CONDUCTING MATERIAL AROUND SAID TRIP, IMPARTING HEAT TO THE OUTER SURFACE OF SAID SHIELD, CONTROLLING THE AMOUNT OF HEAT IMPARTED TO SAID SHIELD INDEPENDENTLY OF SAID CONTROLLED ELECTRIC CURRENT THROUGH SAID STRIP TO MAINTAIN SAID SHIELD AT A TEMPERATURE APPROACHING BUT BELOW THE MELTING POINT OF TIN WHEREBY A UNIFORM MELTING OF THE TIN COATING ACROSS THE WIDTH OF THE STRIP IS ACHIEVED AND CONTROL OF SAID MELTING REMAINS IN THE AFOREMENTIONED CONTROLLED CURRENT, AND QUENCHING THE STRIP IMMEDIATELY AFTER THE TIN COATING BECOMES MOLTEN.

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