US2647304A

US2647304A - Process of terne coating metal and terne coated product

Info

Publication number: US2647304A
Application number: US242496A
Authority: US
Inventors: Nelson E Cook; Samuel L Norteman
Original assignee: Wheeling Steel Corp
Current assignee: Wheeling Steel Corp
Priority date: 1951-08-18
Filing date: 1951-08-18
Publication date: 1953-08-04
Anticipated expiration: 1970-08-04

Description

Aug. 4, 1953 N. E. 309Kv ETAL 2,647,304

PROCESS 0E TERNE coATING METAEAND TERNE COATED PRODUCT Filed Aug. 18, 1951 INVENTORJ' NELsoN E. Coon &

S/)AUELLN-rmu 7 765.52%;

Patented Aug. 4, 1953 PROCESS OF TERNE COATING METAAL AND TERNE COATED PRODUCT e Nelson E. Cook and Samuel-L. Norteman, Wheeling, W. Va.,'assignors 'to Wheeling Steel Corporation; Wheeling, W. Va., a corporation of Delaware Application August 18, 1951, Serial N o. 242,496

This invention relates to the terne coating of metal and to an improved terne coated prod-v uct. It relates particularly to an improvement in the manner of heating a bath of molten terne metal employed for hot dip terne coating whereby important improved results are obtained.

Terne metal comprises lead and tin with lead in preponderance. Normally the percentage of tin is between about fifteen per cent. and somewhat over twenty per cent.; the remainder is substantially all lead except for impurities. g

In the hot dip terne coating of metal the metal to be coated is rst immersed in a ux bath, commonly zinc chloride, after which the metal being coated is immersed in a bathv of molten terne metal in a pot. Prior to our invention the pot has been heated externally to maintain the bath of terne metal molten. Normally the surface of the bath of molten terne metal is divided by a partition member and a bath of oil, usually palm oil or a combination of palm and mineral oils, is oated upon the bath of molten terne metal at one side of the partition member. The metal to be coated is introduced into the bath of molten terne metal at the side of the partition member opposite the side at which the oil bath floating upon the terne metal is disposed. The metal being coated passes through the bath of molten terne metal beneath the partition member and emerges through the oil bath. The use of such an oil bath is well known in the art, the oil acting on the terne metal adhering to the metal being coated to produce a terne coating thereon which is continuous, smooth and desirably free from pin holes.

Optimum results are obtained when the oil of the oil bath has a relatively low flash point. However, the flash point of the oil must be aboveV the temperature of the bath of molten terne metal as otherwise the oil Will ignite andburn. Despite efforts to avoid igniting the oil there have been many occurrences of oil ignition in terne coating pots, some of which have resulted in injuries, serious fires and great damage.

Since it is desirable that the flash point of the oil employed to form the oil bath floating upon the bath of molten terne metal be as low as practicable, the elorts of those skilled in the art have been directed to maintaining assmall a difference between the temperature of the bath of molten terne metal and the ash point of the oil as is consistent with safety. The minimum permissible flash point of the oil has been limited by so-called hotspots or localized zones of undesirably high 'temperature in the molten terne 3 claims. (ci. zei-.194)

"metal which have unavoidably occurred due to the application of heat to the terne metal through the coating pot. It has been necessary to use an oil which will not ash when subjected to the relatively high temperatures encountered at hot spots, and since the maximum temperatures at hot spots are uncertain the applicationv of the necessaryvfactor of safetyhas resulted rin inability to employ oil of desirably low flash point. Despite precautions injuries, serious fires and great damage have occurred and will continue to occur when externally heated terne coating pots are employed.

It is desirable for several reasons to employ a relatively high proportion of tin in the terne metal. The tin aids in bonding the terne metal to the metal being coated, contributes to a sound strong bond, renders the coating desirably fluid as it is applied to the metal being coated and minimizes pin holing which is one of the most common defects in terne coatings. tin has a much lower melting point than lead, so the greater the proportion of tin in the terne metal the lower will be the melting point of the terne metal and consequently the lowerA may be the temperature of the bath of molten terne metal and the lower may be the flash point of the oil forming the oil bath oating on the bath of molten terne metal.

Normally tin is incorporatedv in terne metal to between fteen and twenty-two percent. A rule of thumb as to the composition .ofy terne metal is' that it contains four parts of lead to one part of tin. However, tin is relatively scarce and diflicultly obtainable and for economic reasons it is necessary to reduce the proportion of tin in terne metal to an undesrably low proportion. At the present time government restrictions limit the proportion of tin which may be used in terne metal to ten per cent. One of the most serious disadvantages of the limitationof the proportion of tin in terne metal is that the temperature of the bath of molten terne metal of necessity becomes undesirably high since the temperature of any molten bath is determined by the melting point of the material of which the bath is constituted. This means that the flash point of the oil employed to form the oil bath oatingA upon the bath of molten ternev metal mustlikewise be vundesirably increased. The results are vdiiliculty of operation, ythe production of an inferior product and increased danger of re unless at the higher temperatures employed theiactor o f s afetyras applied to` temperature diierentialbetween/th'e bath oi molten terne Moreover,

metal and the oil bath floating on the bath of molten terne metal is increased, which further detrimentally aiects the process and product.

We have discovered how to obviate the disadvantages above referred to, which disadvantages are real and serious and exist today in all externally heated terne coating pots. We provide 'for maintaining the terne metal molten by electric induction heating and thereby main.- taining the temperature of the terne metal substantially uniform throughout the bath of molten. terne metal and substantially eliminating hot spots. This makes it feasible to use oil having a materially lower flash peint than could be used with an externally heated, terne coating pot with the attendant advantages above indicated in the coating procedure and improved quality of the product produced. Moreover; because of the relatively uniform temperature of the molten terne metal insured by the use of electric induction heating it is possible without sacrice of safety to reduce somewhat the differential between the temperature of the molten terne metal and the nach point oi. the oil employed. The use ci electric induction heating resultsy in maintenance of the molten terne metal at a remarkably constant temperature with the. result that a product of unprccedentedly uniform superior quality is produced without incurring any more danger of tire than is incurred in the use of externally heated, terne coating pots; indeed, the danger of frref is less even while the. improved results are obtained.

Other details, objects and advantages of the invention will. become apparent as the following description oi 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 a. present preferr-ed method of practicing the invention in which Figure l is a central vertical longitudinal crosssectional view, takenon the line. I-I of Figure` 2,. of hot dip terne coating apparatus for coating strip;-l and Figure' 2 is a cross-sectional view of the a10- paratus shown in Figure L taken on the line H--Ii or" that ligure.

Referring now more particularly to the drawings, there is shown apparatus. for terne coating strip. Similar apparatus except modified as to the means used for passing the work through the pot maybe used for terne coating articles. y

Since our invention does not. lie in the means for passing the work. through the pot the terne coating of stri-p is illustrated purely by Way of example, the method so fas as the invention is concerned being the same when articles are coated.

The strip being coated is designated 2' and the pot is designated generally by reference numeral 3f. The strip is guided for movement through the pot by guide rollsrv 4, 5 and 6, conventional exit rollsl as known to those skilled in the art bei-.ng employed in the usual manner.

As the strip 2 enters the pot 3. it passes through a ux bor.` 8 which may be of conventional. structure containing a bath of ux, such as zine. chloride. The strip before passing through the ux is desirablypretreated and washed as known to those skilled in the art,

The 'Strip after passing through the flux in the uur box. 8 is immersed in. abate c. of, molten terne metal contained in the pot 3. The pot 3 `is supported by a framework lll and comprises upper portion thereof as shown in Figure 1 and extends outwardly and downwardly. The cross section of. each passage l5 preferably decreases from relatively great cross section inwardly of thepot to relatively small cross section outwardly of the pot. Each passage I6 communi- Cates with three outwardly and downwardly directed ducts Il which at their outer extremities enter a cross duct t8. Disposed in 2 closed loop about the central one of the three ducts Il is a magnetic core 49 having formed on each of the legs thereof extending between the central duct Il and an outer duct l1 a coil 29 to which electric current is conducted. The current conducted to the coils 2U may, for example,` be 440 volt, single phase, 60 cycle alternating current. There may be provided a pack ofV static capacitators for power factor correction, contactors for switching from high to low voltage for temerature control, a circuit breaker and auxiliary equipment for automatic control of power inputto maintain a desired temperature in the body of the molten terne metal 9.

A low voltage is induced across the ducts l'l. The apparatus is analogous to a step-down transformer having a short-circuited secondary which is formed by molten metal filling the ducts ll and [8. Since the ducts il and i3 form a closed circuit a current will How in them. The current generates heat in the terne metal itself which is the heat maintaining the bath of terne metal molten.

The heat generated in the metal causes a continuous and rapid thermal circulation of the molten terne metal across the pot which maintains the bath of molten terne metal at substantially uniform temperature throughout the bath. It inhibits the formation of hot spots in the bath and enables regulation of the bath temperature to an extent impossible when the pot is heated externally. Because of the close regulation and maintenance of a substantially uniform tempera.- ture throughout the bath of molten terne metal we are enabled to use with safety on the surface of' the molten terne metal an oil having a much lower flash point than could be used otherwise with a significant improvement in the quality of the coated product.

A partition, member 2l extends across the pot substantially at right angles to the axis of the pot which joins the two passages lli so as to intersect the surface of the bath of molten terne metal 5 the bottom 'of the partition member 2l being disposed somewhat below the surface of the molten terne metal.' The strip 2 is immersed in the molten terne metal 9, being introduced thereinto to the left of the partition member 2l viewing Figure l. The strip passes beneath the partition member 2 l and after passing about the guide rollv EY and between the exit rolls i is with` drawn from the pot to the right of the partition member 2| viewing Figure 1. Disposed atop or floating upon the bath of molten terne metal to the right of the partition member 2l viewing Figure 1 is a bath of oil 22, which is the bath of oil above referred to and which has a flash point somewhat higher than the temperature of the bath 9. The bath 9 is substantially free from hot spots so there is no danger of igniting the oil bath 22 through localized increase of its temperature to or above the flash point of the oil. The heating of the bath of molten terne metal by electric induction heating also results in longer pot life and the formation of less scrui. Scruff is formed by deterioration of the pot by the molten terne metal, and we inhibit such deterioration. Moreover, the heating of the bath of molten terne metal by electric induction heating results in superior mixing of the constituent metals and inhibits settling out of the lighter metal on top of the heavier metal. These factors further contribute to a greatly improved product and the obtaining of optimum conditions with a uniformity and regularity not possible with an externally heated pot.

The specific gravity of the molten terne metal 9 is, of course, very much higher than the specific gravity of either the flux in the flux box 8 or the oil 22. The flux is designated 23, its upper surface in the flux box being shown at 24. The level of the molten terne metal 9 to the left of the partition member 2| viewing Figure 1 and outside the flux box is shown at 25 while the level of the molten terne metal 9 within the iiux box Where the Weight of the flux is superimposed upon it is shown at 26. 22 is shown at 21 and the surface of the molten terne metal 9 t0 the right of the partition member 2| where the oil 22 is superimposed upon the molten terne metal is shown at 28.

While we have illustrated and described a present preferred method of practicing the invention 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.

We claim:

1. In the terne coating of metal in which metal is immersed in a bath of molten terne metal and removed from the bath of molten terne metal through a bath of oil floating atop a portion The surface of the oil of the bath of molten terne metal and in which the temperature of the molten terne metal approaches the flash point of the oil so that there is danger of igniting the oil by so-called hot spots or localized zones of undesirably high temperature in the molten terne metal, the step of maintaining the terne metal molten by electric induction heating and thereby maintaining the temperature of the terne metal substantially uniform throughout the bath of molten terne metal and substantially eliminating hot spots whereby to obviate the danger of igniting the oil.

2. In the terne coating of metal in which metal is immersed in a bath of molten terne metal containing not over about 10% tin with the remainder being substantially all lead except for impurities and the metal being coated is removed from the bath of molten terne metal through a bath of oil floating atop a portion of the bath of molten terne metal and in which the temperature of the molten terne metal by reason of the presence of an undesirably small proportion of relatively low melting point tin and an undesirably great proportion of high melting point lead is higher than optimum terne coating temperature and approaches the flash point of the oil so that there is danger of igniting the oil by so-called hot spots or localized zones of undesirably high temperature in the molten terne metal, the step of maintaining the terne metal molten by electric induction heating and thereby maintaining the temperature of the terne metal substantially uniform throughout the kbath of molten terne metal and substantially eliminating hot spots whereby to obviate the danger of igniting the oil.

3. A terne coated product comprising a base metal having thereon a coating of terne metal of improved uniformity and quality applied by the process of claim 1.

NELSON E. COOK. SAMUEL L. NORTEMAN.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,108,381 McElhaney et al. Feb. 15, 1938 2,520,349 Tama Aug. 29, 1950