US2647305A

US2647305A - Process of tight coat hot dip galvanizing and hot dip galvanized product

Info

Publication number: US2647305A
Application number: US231714A
Authority: US
Inventors: Nelson E Cook; Samuel L Norteman
Original assignee: Wheeling Steel Corp
Current assignee: Wheeling Steel Corp
Priority date: 1951-06-15
Filing date: 1951-06-15
Publication date: 1953-08-04
Anticipated expiration: 1970-08-04

Description

Ill-I Aug. 4, 1953 Original Filed Sept. 22

N. E. cooK ETAL 2,647,305 PROCESS OF TIGHT COAT HOT DIP GALVANIZING ANILSHOT DIP GALVANIZED PRODUCT 2A Sheets-Sheetl l Fig. 2.

// /l 1/ /l I/ INVENTORS VNELSON E. COOK nn SAMUEL L. NORTEMAN A4 All@ 4, 1953 N. E. cooK ETAL 2,647,305

PROCESS OF TIGHT COAT HOT DIP GALVANIZING AND HOT DIP GALVANIZED PRODUCT 2 Sheets-Sheet 2 Original Filed Sept. 22, 1950 INVENTORS NELSON E. COOK and.

S L. NORTEMAN Patented Aug. 4, 1953 PROCESS OF TIGHT COAT HOT DIP GAL- VANIZING AND HOT DIP GALVANIZED PRODUCT` Nelson E. Cook and Samuel L. Norteman, Wheeling, W. Va., assignors to Wheeling Steel Corporation, Wheeling, W. Va.,

Delaware a corporation of Continuation of abandoned application Serial No.

186,218, September 22, 1950.

This application June 15, 1951, Serial No. 231,714

6 Claims.

This invention relates to a process of tight coat hot dip galvanizing by use of a molten coating bath comprising zinc and other metal which tends to separate from" zinc and to a hot dip galvanized product coated in such a bath. This application is a continuation of our copending application Serial No. 186,218, filed September 22, 1950, now abandoned.

In the hot dip galvanizing of metal, such, for example, as ferrous metal strip, sheets, ware and pipe, it is customary to utilize a molten coating bath comprising zinc and other metal which tends to separate from zinc. The other metal may be aluminum, lead, antimony, cadmium, tin, etc., either singly or in various combinations. The object of using in the molten coating bath zinc and other metal as above referred to is to improve the quality of the coating. When the galvanized metal is to be deep drawn or otherwise formed after galvanizing it is important that the galvanized coating be tenaciously adherent to withstand the drawing or forming operation without rupture or peeling. The galvanized coating must present a uniform clean pleasing appearance; for certain purposes it should be Spangled These various results are accomplished by incorporating in the molten coating bath along with zinc as the principal coating metal a relatively small quantity of one or more other metals. For purposes of definition we shall herein refer to each of the special coatings above mentioned as a "tight coat.

For purposes of explanation and illustration we shall describe the invention in connection with the tight coat hot dip -galvanizing of ferrous metal strip which is to be used for deep drawing. In order to form on the strip a tenaciously adherent galvanizd coating there is incorporated in the molten coating bath along with the zinc a small but significant quantity, as known to those skilled in the art, of other metal, of which aluminum may be taken as an example, it being the added element normally employed to obtain a tenaciously adherent coating. It has long been known by those skilled in the art that when aluminum is added in small quantities to zinc in the molten coating bath a tenaciously adherent galvanized coating can 'be formed. However, great difiiculty has been experienced in obtaining uniform optimum results. Sometimes a fully satisfactory tight coat will be formed and at other times without apparent change of conditions a coating which, will rupture and peel off when the galvanized metal is deep drawn is produced. The

reason for the failure to obtain uniform optimum (Cl. .Z9-196.2)

i results apparently is that the aluminum or other added metal tends to separate from the zinc in the galvanizing pot s-o as to form strata of metals of different composition. For example, aluminum tends to oat on top of zinc. To obtain satisfactory results it is necessary to continually agitate the molten coating bath to keep the aluminum uniformly admixed with 'the zin-c. Agitation of the bath by thermal circulation brought about by the application of heat externally of the galvanizing pot or by the passage of the work through the pot does not solve the problem. It has been proposed to mechancially stir the, molten coating bath. Mechanical stirring is not, however, a fully satisfactory answer to the problem. The stirring mechanism must be disposed so as not to interfere with the mechanism for moving the work through the galvanizing pot with the result that individual stirrers maintain localized portions of the molten coating bath agitated but do not maintain a uniform composition throughout the portion of the bath through which the work passes while being galvanized. Attempts have been madeto solve the problem by utilizing an excess of aluminum introduced into the bottom of the molten coating bath in the hope that while melting and separating it might work its way slowly up through the bath, but that likewise is 'nota satisfactory solution because the aluminum rises to the top very rapidly and immediately oxidizes, forming an excess of oxides and scruii" which accumulates in the molten coating bath, requiring frequent removal. Moreover, the cost of the additional aluminum is substantial and renders such an attempted solutionl to the problem not economically feasible. Prior to our inventionl no fully satisfactory solution to the problem had been found.4

We have discovered how to obtain avuniform optimum tight coat without the use of mechanical stirrers in the galvanizing pot and without using an excess of aluminum. We have discovered that an unprecedentedly uniform composition can be maintained inthe portion of the molten coatingv bath through rwhich the Work passes while being galvanized through the yexpedient of maintaining the coating bath molten by electric induction heating. Electric inductionheating induces` thermal circulation in lthe bath and 'maintains the molten metal of the bath of relatively uniform composition throughout at least the portion of the bath through which the work moves Ain being galvanized.

It is known that electric induction heating minimizes dross formation but prior to our'invention electric induction heating of the molten coating bath was not seen to have any relation to obtaining a tight coat. We have found that electric induction heating of the molten coating bath maintains the composition of the bath unprecedentedly uniform at the portion of the bath through which the work passes in being galvanized. Electric induction heating obviates the inadequacy of stirrers which arises through the necessity of positioning the stirrers so as not to interfere with the means for passing the Work through the bath. We nd that when the bath is maintained in agitation through thermal circulation by the use of electric induction heating portions of the bath Which WOuldy be inaccessible to stirrers are not inaccessible to the circulation currents brought about by electric induction heating and through suitable positioning of electric induction heating means the molten coating metal throughout the p01'- tion of the bath through which the work passes in being galvanised can be maintained in substantially uniformly admixed condition at all times, thereby insuring a uniform optimum tight coat.

The unprecedented advantage of electric induction heating in tight coat galvanizing is manifested in another way. During operation of a tight coat galvanizing pot the tendency is for the molten coating metal to become relatively rich in Zinc and relatively lean in the added coating metal and quantities of the latter must be added from time to time to maintain the desired proportion. `On occasion the metal coating bath may become suiiciently lean in the added metal that a slightly detrimental effect on the coating can be noticed. Thereupon aluminum or other added metal in solid form is immediately introduced into the metal coating bath. Prior to our invention a considerable length of time was required to melt the added solid metal and distribute it throughout the bath sufiiciently to inhibit the undesirable eiect. We find that when the bath is heated by electric induction heating means the added solid metal is melted and relatively uniformly distributed throughout at least the portion of the bath through which the work passes in being galvanized in a small fraction of the time previously required. This we believe to be because of the unprecedented mixing eiiciency of electric induction heating. It induces thermal circulation not only in portions of the bath spaced from the work handling mechanism at which stirrers can be used but also at portions of the bath in intimate relationship to the work handling mechanism where stirrers would not be effective so that no localized inadequately mixed' spots occur. Mechanical stirring has the fatal defect that stirrers are incapable of operating at the very place where they are most needed, to wit, in the interstices of the mechanism which conveys the Work through the molten coating bath. The work being conveyed through the bath by such mechanism necessarily passes in intimate contact with the mechanism and through such interstices so it is apparent that when mechanical stirrers can only stir other portions oi the bath they are inadequate for maintaining optimum admixture of the components of the bath where most needed. The opposite is the case with .electric induction heating as explained above. We obtain continuously uniformly optimum results.

`Other details, objects and advantages of the invention will become apparent as the 'following description of a, present preferred method of practicing the same and of apparatus upon which the invention may be practiced proceeds.

In the accompanying drawings We have shown apparatus upon which our invention may be practiced and have illustrated certain present preferred methods of practicing the invention in which Figure 1 is a vertical longitudinal cross-sectional view taken on the line I-I of Figure 2 of electric induction heated tight coat hot dip galvanizing apparatus;

Figure 2 is a cross-sectional view o-f the apparatus shown in Figure 1 taken on the line II--Il of that figure;

Figure 3 is a vertical longitudinal cross-sectional view taken on the line III-III of Figure 4 of another form of electric induction heated tight coat hot dip galvanizing apparatus; and

Figure 4 is a cross-sectional view of the apparatus shown in Figure 3 taken on the line IvV-IV of that figure.

Referring now more particularly to the drawings, there is shown in Figures 1 and 2 tight coat hot dip galvanizing apparatus for the continuous galvanizing of strip. The apparatus shown in Figures 1 and 2 will be described as employed in the continuous tight coat hot dip galvanizing of ferrous metal strip intended for deep drawing.

The apparatus of Figures 1 and 2 comprises a galvanizing pot or tank designated generally by reference numeral 2 comprising an outer steel shell 3, a lining 4 of insulating material within the shell 3 and a refractory inner lining 5 for containing the molten coating bath. rlhe molten coating bath for tight coat hot dip galvanizing oi strip comprises zinc and a metal which tends to separate or stratify from the zinc n the pot, commonly aluminum. The aluminum is introduced in very small quantities in proportion to the zinc. lt has a specific gravity less than the specic gravity of zinc and tends to rise to the top of the molten coating bath in the pot and iioat thereon. The result is a tendency toward leanness of the molten coating bath in aluminum except at the surface of the bath with a consequent failure of the aluminum to efficiently perform its intended function of producing a tight coat. Moreover, the aluminum floating on the surface of the bath is rapidly oxidized so that heretofore it has been necessary to frequently add aluminum to make up for the loss of aluminum through oxidation. This has been true even with mechanical stirring of the molten coating bath.

We provide on the galvanizing pot a lateral extension designated generally by reference numeral 6 which has a refractory wall 'I communicating with the refractory lining of pot proper which forms a passage t which entends laterally and downwardly from a side of the pot. The passage 8 communicates with the interior o the pot at the upper portion thereof as shown in Figure 1 and extends outwardly and downwardly. Its cross section preferably decreases from relatively great cross section where it enters the pot to relatively small cross section at its outer portion. The passage 8 communicates with three downwardly and outwardly directed ducts G which at their outer extremities enter a cross duct l0. Disposed in a closed loop about the central one of the three ducts 8 is a magnetic core l I -having formed on each of the legs thereof extending between the central duct 9 and an outside duct 9 a coil l2 to which electric current is conducted. The current conducted to the coils I2 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, contactorsfor 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 thepot.

` A low voltage is induced across the ducts 9.

'The apparatus is analogous to a step-down transformer having a short circuited secondary which is formed by molten metal filling the ducts 9 and l0. Since the ducts 9 and l0 form a closed circuit a current will flow in them. The current generates heat in the metal itself which is the heat maintaining the coating bath molten.

The heat generated in the metal causes a continuous and rapid thermal circulation of the molten coating bath across the pot as shown by the arrows in Figure 1. This circulation maintains the molten coating bath of relatively uniform composition throughout the portion ofthe pot through which the strip passes lin being galvanized, It inhibits separating out orstratifying of metals of different specific gravities with the disadvantages explained above which have occurred throughout the history of the art prior to the present invention and which those skilled in the art were not able to obviate.

A strip i3 which may be of steel is guided so that it can be drawn continuously through the molten coating bath in the pot. A guide roll I4 is disposed beneath the surface of the molten coating bath while other guide rolls l5 and I5 are disposed generally above the galvanizing apparatus. The strip passes generally from left to right viewing Figure 1 and as it emerges from the molten coating bath it passes between coating rolls Il each provided with a scraper I8. The roll M and the strip are omitted in Figure 2 for clarity.

Strip thus continuously galvanized is found to be definitely superior to any continuously galvanized strip previously produced. The coating is found to be relatively tenaciously adherent and even when the strip is heavily galvanized resists severe deep drawing.

Figures 3 and 4 show a form of galvanizing apparatus which is the same as the form shown in Figures l and 2 except that it is adapted for the galvanizing of ware or individual fabricated articles rather than for the continuous galvanizing of strip. Parts shown in Figures 3 and 4 which are the same as parts shown in Figures 1 and 2 are designated by the same reference numerals. The pot of Figures 3 and 4 has, however, a division plate I9 extending completely thereacross separating the surface of the body of the molten coating bath into segregated areas at opposite sides thereof. Flux is floated on one of those areas, the articles to be galvanized, indicated diagrammatically at 2l, being introduced into the molten coating bath through the flux 20, passed beneath the division plate IS and removed from the molten coating bath at the opposite side of the division plate. The ware may be handled by any suitable means as well known to those skilled in the art.

In the so-called hand dipping of fabricated ware the uniformity of the coating and the appearance of the finished galvanized product are vanized by our process as above explained.

While we have illustrated and described cer-4 tain present preferred methods of practicing the Ainvention it is to be distinctly understood that the invention is not limited thereto but may be otherwise variously practiced within the scope of the following claims. y

. We claim: e f l f v1. A processof hot dip galvanizing by use of a molten coating bath comprising molten zinc and at least one other molten metal which tends to separate from molten zincto provide a product having a tight galvanized coating which will not flake off when the product is deep drawn comprising maintaining molten by electric induction heating a coating bath comprising zinc and at least one other metal which tends to' separate from zinc and by such electric induction heating inducing thermal circulation of the molten bath whereby to maintain thevmoltenmetal ofV the bath of relatively uniform composition' throughout at least a portion of the bath with the zinc and other metal uniformly admixed, and passing base metal to be galvanized through said portion of the bath whereby to apply to said base metal a galvanized coating substantially uniform over the surface of the base metal in zinc and other metal.

2. A process of hot dip galvanizing by use of a molten coating bath comprising molten zinc and molten aluminum to provide a product having a tight galvanized coating which Will not flake off when the product is deep drawn comprising maintaining molten by electric induction heating a coating bath comprising zinc and aluminum and by such electric induction heating inducing thermal circulation of the molten bath whereby to maintain the molten metal of the bath of relatively uniform composition throughout at least a portion of the bath with the zinc and aluminum uniformly admixed and passing base metal to be galvanized through said portion of the bath whereby to apply to said base metal a galvanized coating substantially uniform over the surface of the base metal in zinc and aluminum.

3. A process of hot dip galvanizing by use of a molten coating bath comprising molten zinc and at least one other molten metal which tends to separate from molten zinc to provide a product having a tight galvanized coating which will not flake olf when the product is deep drawn comprising maintaining molten by electric induction heating a coating bath comprising zinc and at least one other metal which tends to separate from zinc and by such electric induction heating inducing thermal circulation of the molten bath whereby to maintain the molten metal of the bath of relatively uniform composition throughout at least a portion of the bath with the zinc and other metal uniformly admixed and continuously passing metal strip to be galvanized through said portion of the bath whereby to apply to the metal strip a galvanized coating substantially uniform over the surface of the metal strip in zinc and other metal.

4. A process of hot dip galvanizing by use of a molten coating bath comprising molten zinc and at least one other molten metal which tends to separate from molten zinc comprising maintaining molten by electric induction heating a coating bath comprising zinc and at least one other metal which tends to separate from zinc and by such electric induction heating inducing thermal circulation of the molten bath whereby to maintain the molten metal of the bath of relatively uniform composition throughout at least a portion of the bath with the zinc and other metal uniformly admixed and passing formed metal articles to be galvanized through said portion of the bath whereby t0 apply to said metal articles a galvanized coating substantially uniform over the surface Oi Said metal articles in zinc and other metal whereby t0 insure formation of an optimum tight coat.

5. A process of hot dip galvenizins by use of a molten coating bath comprising molten zinc and molten aluminum t0 prOvde a product havine a tight galvanized coating which will not flake ofi when the product is deep drawn corn-v prising maintaining molten by electric induction heating a coating bath comprising zinc and aluminum and by such electric induction heating inducing thermal circulation of the molten bath whereby to maintain the molten metal of the bath of relatively uniform composition throughout at least a Portion 0f the bath with the zinc and aluminum uniformly admixed and continuou/,sly passing metal strip to be galvanized through said portion of the bath whereby to apply to the metal strip a galvanized coating substantially uniform over the surface of the metal strip in zinc and aluminum.

6. A hot dip galvanized product comprising a base metal having thereon a tight galvanized coating, said coating comprising zinc and at least one other metal which, in the molten state, tends to separate from the zinc, said coating further having the zinc and other metal substantially uniformly distributed therethrough, said coatinghaving been applied by the process of claim l.

NELSON E. COOK. SAMUEL L. NORTEMAN.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,069,923 Crafts Aug. 12, 1913 1,235,628 Wyatt Aug. 7, 1917 1,313,274 Barros Aug, 19, 1919 2,068,687 Liban Jan. 26, 1937 2,347,298 Tama Apr. 25, 1944 2,520,349 Tama Apr. 29, 1950