US2937108A - Method of tinning steel strip - Google Patents

Description

May 17, 1960 T. c. TOY'E METHOD OF TINNING STEEL STRIP Filed Oct. 21, 1955 v INviN'roR q PM ATTORNEYS 2,937,108 METHOD OF TINNING STEEL STRIP Trevor Cyril Toye, Llansamlet, Swansea, Wales, assignor to The British Iron & Steel Research Association, London, England Application October 21, 1955, Serial No; 542,030 1 Claim. (Cl.117--51) 4 This invention relates to the coating of materialsin strip or other continuous or elongated form. The invention is particularly concerned with a method Ofcoating metal in such form, which for the sake of convenience. will be referred to as strip, with a second metal" from a liquid source and this method will be referred to here inafter as A coatingmethod of the type descril' ed.v

v In the case of the tinning of steel strip, an electrolytic process is commonly employed but this has proved to be expensive in practice. The passing of the strip through a bath of molten tin and thence controlling the thickness by nip rolls in'a grease pot as now practised has been found unsatisfactory for continuous strip working because of difiiculties in maintaining the uniform thin coating of tin required, particularly when processing wide strip. The disadvantages of these. processes'apply, although to a lesser extent, als'o'to coating with zinc and aluminum. 7 V

In a coating method of the type described according to the invention the liquidmetalis brought from the source into contactwith the strip by. applying means which are arranged so that during thetransfer of the, liquid metal to the strip relative motionoccurs between a liquid metal applying surface and a liquid metal receiving surface; the strip becomes coated during passage bath to where the strip is in contact with the roller. In addition, by arranging that the strip speed is less than the peripheral speed of the roller, 21 thick coating can be built up. This is of use when the liquid metal is zinc or aluminium.

j. To reduce still further the thickness of the coating and 1 to reduce excess coating material, the coated strip may be heated to near the melting temperature of the coating material and passed through a pair of nip rolls. At least the external parts of these nip rolls are made of a resilientmaterial capable of withstanding the temperature and other working conditions encountered in practice.

For example, suitable silicone rubbers such as that known as Silastomer may be employed.

The invention will be more readily understood by way of example. from the following description of methods of "tinning steel .strip, reference being made to the accompanying drawings, in which Figure 1' illustrates theuse of a roller for applying the tin to both surfaces of the strip,

" Figure 2 illustrates a modification for tinning both surfaces of the strip,

I Figure 3 'shows a further preferred modification, and

Figure 4 illustrates the useof a nozzle for tinning,

i Referring to Figure '1, the strip 12 is drawn under tension froman uncoiler 13 and through the vapour degreaser 14 which incorporates the usual, cooler 15 and which removes oil layers on the strip. The strip is guided by' the guide rollers 16 andv the contact roller 17 and enters duct 18 through an asbestos stuffing gland 19 which acts as a gas seal. The duct 18is secured, gas tight to I a chamber 20 into which the strip passes. After enter past the applying means and according to.the directio n ofthe relative motion, the liquid metal is drawn'out to a very thin film oris caused to build up to a'thick film. The applied metal solidifies onfthe strip and isv held in the thin, drawn out condition or in the thick condition. In this method the strip is not in contact with the liquid coating material for as long as in hot dip coating methods and therefore there is much less alloy formation between the strip and the coating material.

In one form of the invention, the strip which may be steel strip, is drawn past the mouth of a wide thin nozzle supplied from the source with the coating material, which may be molten tin. A liquid tin meniscus is formed between the nozzle and the moving materialso that the coating material is drawn out as a thin film which solidifies on the surface of the strip, the nozzle being supplied from the source through a tube'by capillary action.

'In another form, there is provided a driven roller which dips into the source in theformof a bath of the molten coating material. The strip passes over and in contact with the roller but does not itself dip into the bath. The strip is drawn past the surface of the roller so that the strip moves faster than the peripheral speed of the roller. Again, therefore, drawing out of the coating occurs with an accompanying reduction in coating thickness. In this form of the invention the coating thickness can be controlled not only by the relative speed of roller and strip due to the drawing out action described, but also by the absolute value of the roller speed, the roller diameter, and the depth to which the roller is immersed in the bath of coating material, since these factors will affect the amount of material actually transferred in a given time from the ing the chamber 20,. the strip passes over in sequence a. contact roller 21, a tinning roller 22, a guide roller 23 and a second tinning roller, 24 before leaving the chamher to the duct 25. a v V Thetinning rollers '22, 24 apply tin to opposite sides of the strip and each dips into a bath 26 of liquid tin enclosed in the housing 23. Rollers 22, 24 are driven at peripheral 'speedsless than the speed at which the strip is drawn through the apparatus by a coiler 27. As a consequence, the tin picked up by each tinning roller is drawn out bythe strip from themeniscus formed between the tinning roller and the.strip.. Efiectively, the coating thus applied to the strip by the rollers 22, 24 is stretched andis solidified on the strip in the stretched condition to obtain a very thin coating. The stretched coating. which is unstable is rapidly solidified on the strip to avoid dewctting by quenching in the waterquench 28 into which duct 25 leads. Quenching may however also be effected by submitting the coated strip to a steam jet. applied through inlets 29, 30 in the duct 25; it may also be accomplished advantageously by using an emulsion of 3 parts cotton seed oil. in 1000'parts water in the quench 28, the emulsion also being sprayed on to the strip through inlets '29, 30. I

After being quenched, the strip passes between therubber rollers 31, 32 which rolls off excess emulsion if an emulsion quench has been used, and through the duct 33 through which air is drawn by an evacuating pump at 34 and supplied with warm 'air through 35 and which causes evaporation of any water. remaining on the strip. The strip is finally coiled on the coiler 27.

A heavy alternating current is passed through the strip while in the duct 18 by 'means of the contact rolls 1'], 21. This current raises the temperature of the strip in ducting 18 to about 900 C.,' the temperature being measured by .a pyrometer =at'36. The temperature in the housing 20 itself is maintained at about 300 0.,

this relatively low. temperature being possible'by using:

. ,Patented May 17, 1960..

18 through pipe 37 and out of the duct 25, through pipe 38, preferably consists of equal parts of hydrogen and nitrogen which reduces oxide on the strip and at the could not be lowered below 40D? C. The'operati'ng temperature in the housing 20, prevents excessive FeSnformation and is checked by thermocouples (not shown) bearing against the surface of the roller 23 and in, the baths 26. The pressure of the gas in the housing 20 in conjunction with a flap 40 where duct 25 joins the housing 20 prevents the entry of steam, if steam is used for quenching, into housing 20.

The tinning rollers 22, 24 are preferably made from hard steel or a metal which forms a solid sol'ution'with the tin so that they are wetted by the tin. Suitable materials are tungsten or molybdenum. Possibly, the rollers could be alternatively made of a ceramic which is wetted by liquid tin.

The thickness of the coatingsv applied by the tinning rollers 22, 24 to the strip may be varied by changing the roller diameter or the depth of immersion of the rollers in the baths 26, as well as by variation of the relative speeds of the strip and the rollers. The thickness of the coatings applied to the two surfaces of the strips need not of course be the same. Furthermore, if it is desired to apply athick coating to the strip, and this becomes importantwhen a zinc or aluminium coating is to he applied, the rollers 22, 24. may be caused to rotate at a peripheral speedv greater than the speed of the strip; a thick coating or coatings is then built up. The apparatus herein described is however particularly applicable to tinning.

The speed of rotation of the tinning rollers is limited by the affect of centrifugal motion on the tin transferred by the rollers. Using the apparatus. illustrated in Figure 1 with rollers. 22, 24 of 2 inch diameters, speeds of 25 ft. per minute for the strip have been achieved safely. It is thought that speeds up to 100 ft. per minute with adequate coating thicknesses can be reached. At such high speeds, degreasing of the strip should preferably be effected in a separate preliminary step since the degreaser 14 imposes speed limitations on the strip.

Instead of having the rollers. 22,. 2,4 spaced along the strip path, they may be located adjacent to one another with the strip 12 passing between them. as illustrated in. Figure 2.. The, liquid tin meniscus which is drawn out by the strip is shown. at 41. As before, the rollers 22, 24 may be. driven at the same speed but different from the strip speed, to give equal weights of coating on the two sides of the strip,,or at. different speeds. to give different weight coatings.v

A slight disadvantage of the tinningv arrangements illustrated in Figures 1, and 2 is that there is the danger that the relative movement between the strip 12 and the rollers 22, 24 may cause the strip to become scratched. To avoid this possibility, two rollers may be used for the transfer of tin from the bath 26 to a side of the strip. This is illustrated in Figure 3 in which the rollers 50 dip into the baths 26, transfer the tin picked up tothe rollers 51 which in turn transfer the tin to the strip 12. In this case. no slipping between the rollers 51. and the strip 12 occurs, so. that the danger of. scratching is avoided and the differential movement occurs between rollers 50 and rollers. 51.. For tinning, the rollers 51 have a higher peripheral speed than rollers 54) so that the tin is transferred to rollers 51 in a thin, film which in due course is applied to the strip 12. when a thick coating is desired, rollers 51 are driven at a lower peripheral speed. than rollers50.

In the; alternative arrangement illustrated in Figure 4, the rollers, 22, 24 are; each replaced by a capillary nozzle 52 which extends at its lower end into a liquid tin reservoir 53. The nozzle has a channelv leading, to an.

opening 54 at its upper end, which opening has a width slightly exceeding the width of the strip. The strip is drawn along the top of the nozzle in contact with the liquid tin meniscus 55 and causes the liquid tin to be drawn out of the nozzle as a thin coating on the strip. The liquid tin rises up the nozzle 52 by capillary action, the length and thickness of the channel being chosen in accordance with the desired flow rate as determined by the desired speed for the strip and the coating weight.

The material of the nozzle 52 is a suitable refractory material that will not cause pollution of the tin and is preferably one which is wetted by the liquid tin; if the material is not so wetted the tinmust be supplied to the opening 54 by a pressure feed rather than by surface tension.

In the arrangement of Figure 4, the molten tin is exposed only over a small area at the point of tinning. As the danger of oxide formation is thus small, the provision of a reducing 'atmosphere may not always be required'. Further the danger of contamination. of the. molten tin is minimized and the possibility of accumulation of'scr utf and other solid matter is reduced. The heat losses from the tin are small and the thickness of the coating applied to the strip may be maintained at a.

very small value by suitableadjustment of the speedof the strip and the pressure of the molten tin.

The thickness of the. tin coating. applied to the strip 12 may be reduced thus further by. the provision of a pair of nip rollers designed to remove any excess of. tin. These nip rollers (60, 61) if provided are located between the ducting 33 and the coiler 27 (see Fig. 1). The nip rollers, between whichthe strip passes, operate in a bath of a liquid at a temperature slightly above the melting temperature of tin; for example, the rollers may rotate in a bath of palm oil. Alternatively the rollers may rotate in air in a heated atmosphere 62. The rollers themselves have a resilient surface and a suitable material is a silicone rubber such as that known as Silastomer. These silicone rubbers can withstand. temperatures of about 250 C.,. the temperature at which. they baths are operated, while retaining their resilience. If desired, the Silastomer coating. may have applied. thereto a thin and resilient metal or suitable plastic covering such as polytetrafluorethylene sold under the name Fluon which will protect the Silastorner from. chemical attack by any liquid medium. inthe bath, while stillv providing. the surface resilience required to permit intimate contact. between the nip roller surface and the strip. 7

The high temperature at which the nip rollers operate causethe coating of tin on. the. strip to melt. The nip rollers thenremoveany excess tin onv the strip and reduce the thickness of the coating to a small uniform value such as: 60 micro-inches or less. The resilience ofv the rollers. ensures an. intimate contact between the rollers and the strip thereby ensuring uniformity in the coating. The. action is then one of mangling which is more effective than when hard, non-resilient rollers are employed. After passing through the nip rollers, the strip may be subjected to radiant heating to reflow the tin.

While not illustrated in any of the figures of the drawings, means: for causing mechanical vibration may be provided at the point of tinning by lateral agitation of the strip as it. passes over the tinning roller or tinning nozzle; such mechanical vibration assists tinning at high strip speeds. and ensures a good bond between the coat-- ing andthe strip and continuity in the coating; the. fre quency of vibration may be-in the ultrasonic range. and. can be generated and applied to the strip by using knownelectromechanical. transducer elements.

What I. claim is:

A method of. tinning a. longitudinally travelling steel strip which. comprises the steps of heating the strip to a temperature of approximately 900 C.,. cooling the. strip by a. reducing, atmosphere constituted by a. mixture of equal parts of hydrogen and nitrogen at a temperature of approximately 300 C. and simultaneously applying in the reducing atmosphere molten tin upon one surface of the strip across the entire width thereof by means of a first roller which has a metallic surface which forms a solid solution with the tin and which dips into a source of the molten tin and deposits the same on the strip, applying in the reducing atmosphere molten tin upon the opposite surface of the strip by means of a second roller which also has a metallic surface forming a' solid solution with the tin and which dips into a source of molten tin and deposits the same on said opposite surface of the strip, adjusting the peripheral speeds of said first and second rollers to difierent values greater than the speed of said strip so that tin layers of different but predetermined thicknesses are applied to the opposite surfaces of the strip, and passing the coated strip between a pair of nip rolls which are maintained at a temperature slightly above the melting point of tin to thereby remove excess tin from said strip.

References Cited in the file of this patent UNITED STATES PATENTS Taylor Sept. 9, Bundy Dec. 23, Palm Apr. 24, Supligeau et a1 Sept. 10, Simmons Oct. 12, Crew- May 17, Reilly Aug. 12, Lignian Aug. 10, Buck et a1. Sept. 7, Fink May 1, Lathem et al. Oct. 31, Jenne Jan. 9, Nordquist May 27, Butler Aug. 10, MacDonald Nov. 23, Harmon Feb. 12, Hahn Feb. 4,

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