US3066041A

US3066041A - Method of hot-dip metallising metal strips

Info

Publication number: US3066041A
Application number: US45130A
Authority: US
Inventors: Busch Andreas
Original assignee: Stahl & Walzwerke Rasselstein
Current assignee: Stahl & Walzwerke Rasselstein; STAHL- und WALZWERKE RASSELSTEIN/ANDERNACH AG
Priority date: 1959-07-29
Filing date: 1960-07-25
Publication date: 1962-11-27
Anticipated expiration: 1979-11-27

Description

Nov. 27,1962 A. BUSCH 3,066,041

METHOD OF HOT-DIP METALLISING METAL STRIPS Filed July 25. 1960 4 Sheets-Sheet 1 INVENTOR.

AND/PEAS 5115c ATTORNEYS Nov. 27, 1962 A. BUSCH 3,066,041

v METHOD OF HOT-DIP METALLISING METAL STRIPS Filed July 25, 1960 4 Sheets-Sheet 2 FIG}.

HVTOR. AND/F545 BUS C A/ A TORNEYS Nov. 27, 1962 A. BUSCH 3,066,041

METHOD OF HOT-DIP METALLISING METAL STRIPS Filed Jul 25, 1960 v 4 Sheets-Sheet :5

,p o c) 0 INVENTOR.

A/Vfl/PEAS 5056 wwawwwgm.

ATTORNEYS 4 Sheets-Sheet 4 mmvron 1/1/0254: 3050/ A. BUSCH Nov. 27, 1962 METHOD OF HOT-DIP METALLISING METAL STRIPS Filed July 25, 1960 H A: w 1

ATTORNEYS he States In the known procedure for hot-dip metallising strips of steel or metal, the steel strip passes first through a flux and then through a metal melt, for instance of tin, whereafter the surplus metal is retained by means of squeezing rolls which run in a grease bath placed immediately above the metal vat. This known procedure has considerable disadvantages, more particularly so far as hot-dip tinning is concerned. Out-put is very low, since a strip can pass through the tin bath and through the squeezing rolls, which run in palm oil, at a speed of only about 2.4 m./min. The speed cannot be increased; if it is, the thickness of the tin coating becomes excessive. The thickness of the tin cannot be controlled accurately and reliably. The tin layer is relatively thick and varies over a range of about -40 g./m. The thickness of the tin is uneven and a variation of i3 g./m. is likely in a strip. The tin layer is also nonuniform-i.e., porous-so that hot-diptinned strips or plates do not resist corrosion well. Another disadvantage is that a number of pairs of squeezing rolls running in palm oil must be provided. The squeezing rolls must be made of steel, and the roll periphery becomes covered with a tin film, the thickness of which governs the thickness of the tin layer applied to the strip. Also, the steel strips has to be given a complex degreasing treatment after being tinned.

The alternative methods which have been proposed for hot-dip metallising have not led to any satisfactory result. It is an object of this invention to provide a considerable increase in the speed of the strip being treated, and consequently to improve output considerably. Another object of the invention is, notwithstanding a high strip speed, to reduce the thickness of the metal coating or to provide selective control of such thickness. A final object of the invention is, despite a high strip speed, to provide satisfactory, uniform and non-porous metal coverings. A high strip speed means considerable difficulties in completely removing surface impurities, more particularly metal oxides. According to an earlier proposal by the applicants which has not previously been published, the strip, which is with advantage preheated, is first given conventional hot-dip metallising treat1nenti.e., it is passed through a flux bath into a metal melt--whereafter the strip which issues from the melt passes through squeezing rolls which preferably have a metal-repellant surface, such rolls running exclusively in a protective gas atmosphere and being supplied, before the strip enters the roll gap, with a reducing agent, preferably a gaseous flux. This method enables the strip to move very fast, for the squeezing rolls do not run in a liquid. However, after some time has elapsed the squeezing rolls become soiled by the metal oxides or reduction residues carried along by the strip, with the result that the strip surface is impaired. One reason for this is that the strip moves so fast through the flux bath that the same cannot completely deoxidise the strip, and reduction residues are carried along through the melt.

To obviate these disadvantages, in the method according to the invention a metal intermediate layer, such as tin, as first applied to the strip. Then the strip is heated to a temperature above the melting point of the intermediate layer, preferably in an inert or reducing atmosphere.

3,6dtifil4l Patented Nov. 27, 1962 ice Then the strip is introduced into a metal melt, such as tin, and a flux, preferably gaseous, is so supplied to the strip before the entry of the strip into the metal melt that the dissolved impurities, more particularly metal oxides, float away on the concave wetting meniscus formed, by the metal melt on the strip. The impurities are removed continuously, from the strip as it enters the melt. The strip issuing from the melt is conveyed in aprotective gas atmosphere to the squeezing rolls which rolls run ex clusively in such atmosphere. The advantage of this novel method is that the provision of a metal intermediate layer leads to the strip comprising relatively few impurities, more particularly surface metal oxides, and even they can be readily removed according to the novel method when the strip dips into the metal melt at high speed. Another factor enabling the strip to run at a high speed is that the surplus tin is removed from the tin issuing from the bath by means of squeezing rolls which run exclusively in a protective gas atmosphere. If required, the reliability of the novel method can be further improved by supplying the squeezing rolls with a non-liquid and preferably gaseous flux. The squeezing rolls used have a metal-repellent and preferably resilient surface layer. The squeezing roll surface can consist of chromium-plated steel or carbon or porcelain or quartz glass. Strip speeds of about 40 m./min. are possible with such rolls, but if silicone rubber is used for the roll surface, strip speeds of up to m./min. are possible. The roll pressures are much less than the high roll pressures associated with the known squeezing rolls running in palm oil-only about 20 kg. for a roll width of 600 mm.

In the hot-dip leading of steel strips it is known for the strip first to be tinned by galvanising, whereafter the tin layer is fused to the strip in a continuous furnace in a reducing atmosphere, and only thereafter does the strip pass into a lead melt. The pretinning and fusion are necessary in this hot-dip leading method to provide an intermediate layer which consists of an alloy of iron and tin and which must be provided if the layer of the lead is to adhere satisfactorily. In this known procedure the surplus lead is squeezed off the strip by means of squeezing roll pairs running in palm oil, with the result that strip speed is very low. Also, the reducing atmosphere in the continuous furnace does not sufiice to remove the metal oxides from the strip, consequently a flux bath must, as a rule, be provided in conventional manner on the entry side of the lead melt. Even if sufficient pretinning is provided to enable a flux bath to be omitted, continuous operation is impossible. At high strip speeds, the amount of impurities produced is so great that their removal is not technically feasible.

The invention will hereinafter be described in greater detail with reference to the drawings which illustrate embodiments and in which:

FIG. 1 is a diagrammatic view of the complete apparatus for metallising metal strips;

PEG. 2 is a detailed view of the squeezing roller arrangement used in the apparatus shown in FIG. 1;

FIG. 3 is a view to an enlarged scale showing the entry of the strip into the metal melt of FIG. 1;

\FIG. 4 shows a variant of the apparatus in FIG. 3; and

FIG. 5 shows an alternative arrangement for carrying the novel method into effect.

Referring to FIG. 1, a metal or steel strip is unwound from a strip reel 1. The following are arranged in the path followed by the strip from the reel 1 to a taking-on reel 16: a Welder 2; a retarder 3; a pickling device 4; a brushing and washing device 5; a galvanising plant 6 for applying a metal intermediate layer; a brushing and washing device 7; a continuous furnace 8 in which the strip is preheated by heaters 9 and and the metal intermediate layer which has been applied by the galvanising plant 6 is fused to the strip; a metal melt 10 through which the strip passes; squeezing rolls 11 for removing surplus metal; a cooling duct 12 for cooling the strip; a tank 13 for aftertreatment (passivation) of the strip; a drying duct 14 and a drawing device 15. The remainder of the description refers by way of example to the tinning of a steel strip.

The strip is provided in the galvanising plant 6 with a thin tin layer of only about 0.5-2 g./m. thickness (on both sides). This intermediate layer is fused on to the strip in the continuous furnace in an inert or reducing protective gas atmosphere and (FIG. 3) the steel strip 17 coated with this liquid tin intermediate layer 18 is introduced into the tin melt It). Inductive heaters 9 or other means can be used to heat the strip in the furnace S. Conveniently, the inert protective gas is supplied to the bottom of the continuous furnace at 19. The impurities still present on the strip surface, more particularly metal oxides, are indicated by the references 2%). A concave meniscus 21 consisting of liquid metal (It?) forms where the strip dips into the melt 10. Consequently the impurities can float away on this meniscus, in the direction indicated by arrows, on to the surface of the melt 10. To ensure that the impurities, more particularly metal oxides, are completely removed from the strip surface, supply ducts 22 are provided on either side of the strip for a preferably gaseous flux, such as hydrogen chloride, which issues from fine apertures 23-i.e., very near the meniscuses 21. Such flux, rising in ducts 24 on either side of the strip, is indicated by the reference 25. As is apparent from the drawings, the exit duct 24 widens into a hood 27 which dips into the melt 10 at 26, the flux supply ducts 22 being disposed on the hood, preferably in the transition region between the duct 24 and the hood 27. Advantageously, the walls of the exit duct 24 are at a very reduced distance a from the strip 17, and the hood 27 is similarly at the very reduced distance a from the surface of the metal melt. The distance a is from about 3 to 5 mm. Intimate engagement of the flux with the strip surface and the melt is therefore ensured.

The impurities 2% which float off on the meniscus at 21 are removed continuously from the surface of the melt It). To this end (FIG. 3), there are provided on either side of the strip inside the hood 27 a rotating steel roller 28 which dips into the melt, a scraper 29 which is made of asbestos or the like and which co-operates with the roller 28 and a collecting box 30 which also dips into the melt. The rollers 23, which rotate slowly in the direction indicated by arrows, remove the impurities continuously from the melt surface and convey the impurities to the boxes 30.

The hood 27 and duct 24 can be moved vertically relatively to the furnace 8 so that the hood can be lifted in order to be cleaned or for a check to be made of the meniscus 21.

In the novel method there is no layer of flux, such as is usually found, on the metal melt l0, consequently it is easy to check the meniscus 21 to see whether the impurities are being removed satisfactorily. If required, the impurities can be removed from the melt 19 in a different way, for instance, as shown in FIG. 4, where collecting boxes 32 which dip into the melt and which each comprise an overflow 31 for the impurities 2d are provided one each on either side of the strip inside a hood 27a; disposed in each box 32, with the interposition of a filter 33 of glass wool or the like, is a pump 35 which is driven by a motor 34- and which conveys to the melt ltl, through a return pipe 35, the metal which has spilt into the box through the overflow pipe 31. Using this overflow principle, impurities which float off the strip 21 can be removed reliably at high strip speeds.

If required, the method according to the invention can be carried into effect using an arrangement of the kind shown in FIG. 5 wherein a continuous furnace Sa in which the strip 17 moves upwards is provided to fuse the metal intermediate layer 18 to the strip 17. Flux supply ducts 38 are disposed on either side of the exit channel 37 of the furnace 8a, and collecting tanks 39 which receive the impurities 20 floating off the meniscus at 21 are disposed below the ducts Immediately above the duct 37 are supply pipes 49 through which the metal melt 41 is supplied and which extend to near the strip at 42. The melt 41 can be supplied by capillary action, the melt being supplied to the supply pipes 46 from a supply tank 43. Disposed above the pipes 4-1 is a casing which receives squeezing rolls 44 and to which an inert protective gas is supplied through pipes 46. Preferably, a flux, for instance, hydrogen chloride, is supplied in small amounts through pipes 47 to the strip before the same enters the roll gap.

A similar squeezing roll arrangement is provided in the apparatus shown in FIG. 1 and is illustrated in FIG. 2. Referring to FIG. 2, the squeezing rolls 11 run in an inert gas atmosphere in a casing 43. The inert gas is supplied through pipes 49 to a duct 5% which dips pcrpendicularly into the melt ltl. Pipes 51 through which a gaseous flux can be supplied open into the duct 50 immediately below the roll gap.

The quantities of flux supplied at 22 and 38 at the place where the strip enters the melt are relatively small; similarly, the gas pressure in the

pipes

22 or 38 is relatively low and is only about 5-20 mm. water.

The novel method hereinbefore described has the advantage that, nothwithstanding high strip speeds, uniform and non-porous metal coverings can be produced. Fairly thin tin layers of up to 15 g./m. can be provided in hot dip tinning using the novel method. The thickness of the metal layer can be varied as required by varying the surface qualities of the tin-repelling squeezing rolls. Metal coatings of very reduced thickness can be produced by means of polished squeezing rolls.

A great advantage of the premetallising step, which can be formed not only by galvanising but, if required by diffusion or pasting or the like, is that the immersion times in the metal melt can be very short, with the result of a further advantagei.e., a reduction of the alloy layer. Since the immersion times are so very short, the vessels containing the melt It can be very small.

Another advantage of the method according to the inventioni.e. of the premetallising stepis that the melt can consist of metals not alloyable with the basic metal of the strip, since an appropriate metal can be used for the intermediate layer in the premetallising step. In hot dip leading, therefore, the steel strip can be provided with an intermediate layer of tin, while when a steel strip is hot-dip metallised with aluminium, a zinc intermediate layer can be provided. The invention is therefore not limited to the hot-dip tinning which has been mentioned solely by way of example.

I claim:

1. A method of hot-dip metallizing metal strips which comprises: applying a metal intermediate layer to the st ip; heating said strip to a temperature above the melting point of said metal intermediate layer in a nonoxidizing atmosphere; applying a flux to the heated strip and then passing the strip through a metal melt, the molten metal forming a concave wetting meniscus where the strip enters the metal so that the dissolved impurities on the strip are floated away therefrom; and moving the strip exiting from the bath through a protective gas atmosphere to squeezing rolls which operate in the protective gas atmosphere.

2. A method according to claim 1 wherein the flux applied to the strip is in a gaseous form.

3. A method according to claim 1 including the step of applying a non-liquid flux to the strip after it exits from the molten metal and before it enters between the squeeze rolls.

4. A method according to claim 1 wherein the squeeze rolls have a metal repellent surface layer.

5. A method according to claim 4 wherein the surface layers of the squeeze rolls are resilient.

6. A method according to claim 1 wherein the strip is of steel, the intermediate layer consists essentially of tin and the flux is hydrogen chloride.

7. Apparatus for the hot'dip metalization of metal strips, comprising: a furnace through which the strip is forwarded, said furnace having an entrance end and an exit end; flux supply ducts adjacent to the exit end of the furnace for applying a flux to said strip; molten metal applying means closely adjacent to said flux supply ducts for applying molten metal to said strip; means for removing impurities from the zone where the molten metal is applied to the strip; a casing closely adjacent said applying means so that the strip passes directly from said applying means to said casing, said casing having rotatable squeeze rolls therein between which the strip passes; and means for maintaining a protective gas atmosphere in said casing.

8. Apparatus for the hot-dip metalization of metal strips, comprising: a furnace through which the strip is forwarded; a metal bath located below the furnace; an exit conduit extending from the lower end of the furnace toward the bath; a hood connected to the lower end of the exit conduit and extending sidewardly therefrom, said hood overlying the upper surface of the metal in said bath, said hood having edge portions spaced laterally from said exit conduit which edge portions extend downwardly into said bath; means for supplying a flux to the interior of said conduit and said hood, said last-named means being located Where said hood is connected to said exit conduit; and means for supplying a protective gas to the lower end of the furnace.

9. Apparatus according to claim 8, in which the internal walls of the exit conduit and the hood are located close to, but are spaced from, the strip and the upper surface of the metal in said bath, respectively.

10. Apparatus according to claim 9, including a pair of rotatable rollers located underneath and spaced from the hood, said rollers being located on opposite sides of the strip, said rollers being partially submerged in the bath so that the upper portion of the periphery thereof is above the upper surface of the bath whereby impurities on the upper surface of the bath are carried out of the bath on the peripheries of said rollers; a scraper for removing impurities from the peripheries of the rollers; and a collecting box associated with each roller for receiving the impurities removed therefrom by the scraper, the collecting box also extending partway into the molten metal.

11. An apparatus according to claim 9, including a pair of collecting boxes located underneath and spaced from the hood, the collecting boxes being located on opposite sides of the strip, the collecting boxes being at least partially submerged in the bath, said boxes each having an overflow on the surface of molten metal through which impurities on the surface of the bath may enter said collecting boxes; a filter within each of the boxes for separating the impurities from the metal which may flow into the collecting boxes therewith; and a pump for transferring the metal separated in the collecting box back to the bath.

12. Apparatus for the hot-dip metalization of metal strips, comprising: a furnace through which the strip is forwarded, the strip being moved upwardly through the furnace; an exit conduit extending upwardly from the upper end of the furnace; flux supply ducts for supplying flux to said exit conduit; collecting boxes for receiving impurities removed from the strip, said collecting boxes being disposed below the flux supply conduits; metal sup ply ducts located above and closely adjacent the upper end of said exit conduit for supplying molten metal to said strip as it exits from said exit conduit; a casing having rotatable squeeze rolls therein between which the strip passes, said casing being disposed above and adjacent to said metal supply ducts; and means for supplying a protective gas to said casing.

References Cited in the file of this patent UNITED STATES PATENTS 2,216,519 Quarnstrom Oct. 1, 1940 2,224,578 Wean et al. Dec. 10, 1940 2,405,592 Manger et al. Aug. 13, 1946 2,459,161 Harris et al. Jan. 18, 1949 2,656,285 Burns et al. Oct. 20, 1953 2,761,793 Brennan Sept. 4, 1956 2,914,423 Knapp Nov. 24, 1959

Claims

1. A METHOD OF HOT-DIP METALLIZING METAL STRIPS WHICH COMPRISES: APPLYING A METAL INTERMEDIATE LAYER TO THE STRIP; HEATING SAID STRIP TO A TEMPERATURE ABOVE THE MELTING POINT OF SAID METAL INTERMEDIATE LAYER IN A NONOXIDIZING ATMOSPHERE; APPLYING A FLUX TO THE HEATED STRIP AND THEN PASSING THE STRIP THROUGH A METAL MELT, THE MOLTEN METAL FORMING A CONCAVE WETTING MENISCUS WHERE THE STRIP ENTERS THE METAL SO THAT THE DISSSOLVED IMPURITIES ON THE STRIP ARE FLOATED AWAY THEREFROM; AND MOVING THE STRIP EXITING FROM THE BATH THROUGH A PROTECTIVE GAS ATMOSPHERE TO SQUEEZING ROLLS WHICH OPERATE IN THE PROTECTIVE GAS ATMOSPHERE.