Method of and apparatus for the cladding of steel sheet or strip with lower melting metals or alloys

A cladding process and apparatus in which steel strip or sheet is cladded with a metal of lower melting point, e.g. lead, by maintaining a bank of constant level of the molten cladding material between a belt and the strip or sheet substrate which passes along an inclined path and is cooled to harden the molten material thereon. During the cooling process the applied material is compressed against the substrate.

FIELD OF THE INVENTION 
My present invention relates to a method of and to an apparatus for the 
cladding of steel strip or sheet with lower melting metals or their alloys 
and, more particularly, to a cladding process in which a molten metal or 
an alloy is applied to and bonded to the substrate. 
BACKGROUND OF THE INVENTION 
Strip steel or sheet steel can be coated with lower melting metals, e.g. 
lead, by applying the molten metal to the steel substrate and permitting 
the molten metal to cool as a layer on the substrate and bond thereto. 
Methods of cladding steel in this manner generally apply the molten metal 
to the steel strip or sheet substrate while the latter is inclined to the 
horizontal and induce solidification of the coating by forced cooling of 
the substrate and the applied materials. 
In the system of German Pat. DE-PS No. 718,528, for example, the strip or 
sheet is passed through a channel which is inclined to the horizontal and 
into which the molten metal is poured. 
In British Pat. No. 1,356,782, the material is applied from a funnel to the 
inclined substrate. 
Both processes have been found to have a common disadvantage in that the 
feed of the substrate and the cooling of the applied molten material must 
be carefully coordinated and controlled with high precision if defects in 
the coating are to be excluded and malfunctions in the operation of the 
process are to be avoided. 
Another disadvantage, particularly in the case of the system of German Pat. 
No. 718,528, is that the substrate to be coated, or the coating material, 
is limited as to the shape, extent or parameters of the cladding layer 
which can be fabricated. 
It is known also to clad sheet or strip steel with lead (German patent 
document--Open Application DE-OS No. 20 08 454), introducing the substrate 
at an acute angle to the horizontal, from above, into a bath of molten 
lead overlain by a slag layer and then to draw the substrate, with the 
lead adhering thereto, through a die of appropriate shape determining the 
thickness of the cladding layer or layers. 
Experience has shown that lead baths of this type suffer segregation and 
tend to develop inhomogeneities which may affect the coating. 
Consequently, even after only a matter of hours, especially in the case of 
copper-alloyed lead, it is necessary to interrupt the operation, clean and 
empty the entire apparatus and then refill it before beginning again the 
cladding process. 
Because of the time-consuming nature of the cleaning operation and the 
downtime of the apparatus during the cleaning procedure, this system has 
serious economic handicaps. 
If cleaning is not carried out after sufficiently short intervals of 
cladding operation, the segregation brings about nonuniform coatings which 
result in warping and in irregular cladding. 
Furthermore, experience has shown that the apparatus used in this system 
has disadvantages apart from those enumerated above in that, for example, 
it is difficult to satisfactorily seal the die against the leakage of lead 
and to restart the operation after the latter has been interrupted as is 
required after each cleaning sequence. 
Finally, in connection with this arrangement it is noted that the system 
requires strong tensile forces to draw the substrate through the bath and 
the die, these forces frequently giving rise to differential changes in 
length between the lead layer and the substrate causing structural 
complications at the interfaces and in critical zones where bonding is 
required. 
OBJECTS OF THE INVENTION 
It is the principal object of the present invention to provide an improved 
method of cladding steel strip or sheet with lower melting metals or 
alloys whereby the disadvantages of earlier systems are obviated. 
Another object of this invention is to provide an improved method of 
applying coatings of low-melting materials to a steel substrate which can 
be carried out continuously and with a minimum of interruption, yielding 
an effectively bonded coating on the substrate which is homogeneous, 
uniform and free from cracks or like discontinuities. 
Still another object of the invention is to provide an improved but 
relatively simple apparatus for the cladding of steel strip or sheet with 
low melting metals or alloys which permits fine control of the process but 
yet does not require as critical a control of the substrate feed and 
cooling as do conventional processes. 
SUMMARY OF THE INVENTION 
These objects and other which will become apparent hereinafter are 
attained, in accordance with the present invention, in a method of 
cladding steel strip or sheet (hereinafter a steel substrate) with a lower 
melting metal or alloy (material) wherein the substrate is passed at an 
inclination to the horizontal and the molten metal is retained against the 
substrate by an endless belt which, at least over part of its path, is 
parallel to the surface of the substrate to be cladded, the molten 
material being laterally retained by sliding shoes which can rest upon the 
substrate. 
According to the invention, moreover, in the region in which the molten 
material is initially applied to the substrate, a reducing atmosphere is 
maintained, thereby ensuring the monogeneity of the bank of molten 
material which progressively feeds the layer by passing between the 
endless belt and the surface of the substrate juxtaposed therewith. 
The inclination of the substrate is ensured by a roller array upon which 
the opposite surface thereof rests and this array can be adjustable to 
vary the inclination at any angle between 30.degree. to 70.degree. to the 
horizontal although an angle of about 45.degree. is preferred. 
Because the distance between the endless belt and the substrate is 
adjustable, the thickness of the cladding layer can be selected within a 
wide range depending upon market requirements. Preferred thicknesses are 2 
to 20 mm although both smaller and larger thicknesses can be used if 
desired. 
As indicated, between the surface of the substrate to be clad with the 
molten metal and the downwardly turning portion of the endless belt, which 
may be composed of a material unaffected by the molten material and 
nonadherent thereto, there is provided a bank of the molten material which 
is held constant and preferably as small as possible and as close as 
possible to the upper direction-change roller of the belt-guidance system. 
A sensor, e.g. an optical device, can be provided to control the flow of 
molten metal to this bank. 
Advantageously, this upper roller and the bank of molten metal is disposed 
within a hood or other enclosure retaining the reducing gas blanket which 
prevent oxidation of the molten material. 
Advantageously, this reducing gas blanket is maintained by combustion in 
this zone at a substoichiometric ratio with respect to a hydrocarbon fuel. 
Thus if the air/fuel ratio .lambda.=1 represents a stoichiometric amount of 
atmospheric oxygen sufficient to combust completely all of the fuel (to 
CO.sub.2 and H.sub.2 O), we prefer to use an air ratio .lambda. between 
0.90 and 0.95 to maintain a reducing atmosphere. This, of course, 
corresponds to 5 to 10% less oxygen than is required to completely burn 
all of the fuel. 
The combustion is preferably carried out with burners spaced apart across 
the width of the substrate and trained against the molten material which 
is applied thereto. We have found it to be advantageous to feed the fuel 
and air at such velocity that the flame cones from the burners penetrate 
to a depth of 3 to 15 mm (approximately) into the molten material of the 
bank formed on the substrate. 
This additional heating results in a significant improvement of the bond of 
the molten material to the substrate, probably as a result of the 
combination of the additional heating with agitation caused by the jets 
emerging from the burners. 
The length, orientation or position and speed of the endless belt can be 
selected, according to another feature of the invention, so that kneading 
of the molten metal layer and the solidifying layer occurs during the 
solidification process. At least some kneading is desirable when the 
solidification has progressed to the point that the molten material has 
doughy consistency. For example, this may result by making the speed of 
the belt somewhat greater than the speed of the substrate and/or by 
pressing portions of the belt between the direction-change rollers, more 
deeply into the layer than elsewhere. 
The kneeding of the molten material appears to result in a compaction of 
the cladding layer, an improvement in the metallurgical structure of this 
layer and a reduction in structural defects. Cracks which have appeared in 
other cladding systems are excluded with the system of the invention and 
greater structural integrity is assured. 
We have found it to be advantageous to position the sliding shoes which 
laterally confine the layer of molten material between the belt and the 
substrate, so that the cladding is applied short of the edges of the 
substrate, i.e. an uncladded zone is formed along each longitudinal edge 
of the substrate to a width of, say, up to 5 cm. This has been found to be 
particularly desirable when the clad substrate is used in the fabrication 
of larger bodies and welding along the longitudinal edges is desirable. 
The substrate can be pretreated in a conventional manner. For example, it 
can be subjected to a coarse cleaning to remove scale by sandblasting, 
shot peening or steel-grit blasting, the coarse mechanical descaling being 
followed by a pickling treatment preferably in such manner that the 
evolution of hydrogen is excluded or minimized. 
It has been found to be advantageous to spray a liquid pickling agent from 
a nozzle array onto the substrate of a minimum length thereof and to 
thereafter treat the surface with jets of steam to remove the pickling 
solution and effect final cleaning with a minimum tendency to reoxidation 
of the surface. 
In yet another feature of the invention, after the cleaning in the manner 
described, a primer or bond promotor can be applied to the surface of the 
substrate to be cladded with the molten material. The primer, which can be 
a metal alloy, also serves to protect the surface of the steel from 
oxidation. 
The substrate can be supplied to the cladding stage at an elevated 
temperature resulting from the heating during application of the alloyed 
primer although, in any event, it is desirable to heat the substrate to a 
temperature close to that of the molten material before it reaches the 
molten material. This yields a particularly effective bond and adhesion of 
the cladding layer to the substrate. 
After the molten material is applied to the substrate, forced cooling is 
indirectly carried out, e.g. by means of cooled rolls, and most 
advantageously within the zone of contact of the endless belt by directing 
a cooling fluid such as water or compressed air to the uncoated underside 
of the substrate. 
Direct cooling of the endless belt on its side turned away from the 
substrate may also be effected. The belt can thus be composed of a metal, 
e.g. stainless steel, or some other material to which the cladding 
material or alloy does not readily adhere. 
The process of the present invention has widespread application and can be 
used to apply virtually all metals and alloys which have a lower melting 
point than that of steel to the strip or sheet substrate. 
The preferred cladding materials are lead or copper or their alloys and the 
primer can be tin, a tin-lead alloy (solder), if the substrate is to be 
cladded with lead or a lead alloy, or silver solder if the substrate is to 
be cladded with copper or copper alloy. 
When tin or a tin-lead alloy is applied as the primer, a suspension of 
powder thereof in a liquid flux can be applied to the preheated or 
concurrently heated substrate. 
The method of the present invention can be carried out continuously with 
inspection and access to the product during the coating process being 
available at all times. The substrate can utilize practically any width 
and gauge of steel strip or sheet and the process can be carried out with 
a linear speed, depending upon the thickness of the cladding layer, which 
can be as high as 40 m/min.

SPECIFIC DESCRIPTION AND EXAMPLE 
In the drawing I have shown a steel strip 1 which is fed along a transport 
path in which it can be subjected, on its upper surface, to steel-grit 
blasting at 20, a brief spraying of hydrochloric acid as a pickling agent 
at 21 so that hydrogen evolution is minimized or excluded and blowing and 
final cleaning with steam jets 22. 
The strip is then heated by three arrays of burners 2, each spray being a 
row of burners spaced apart across the width of the strip (perpendicular 
to the plane of the paper in FIG. 1)and trained upon the underside of the 
strip. 
The thus-heated strip is then sprayed via a nozzle 23 with an agitated 
tin-lead powder in a liquid flux from a mixing tank 3 to which the liquid 
flux is supplied from a reservoir 5 and the tin-lead is supplied from a 
fluidized bed storage vessel 4, e.g. via a worm conveyor 24 forming a 
metering device. 
The strip is subsequently heated, e.g. by burner arrays 25 to a temperature 
above the melting point of the priming alloy and preferably close to the 
melting point of the cladding layer. 
The strip steel then enters the cladding zone in which it is heated by a 
row of burners 6 to the cladding temperature for the lead (about 
350.degree. C.), the lead being supplied from a molten-lead storage vessel 
7 via a valve 27 responsive to a controller 26 which receives input from a 
sensor 28 detecting the level of the bank of molten layer upstream of the 
endless belt 10 and the direction-change roller 11. 
The supply of the molten material is thus limited to maintain the low bank 
29 thereof. 
Within a hood 9 in this region of the coating device, the burners 8, 
likewise spaced across the width of the strip, are provided to maintain a 
slightly reducing atmosphere which excludes air from the region of the 
hood and prevents oxidation of the molten material. 
The endless belt 10 passes around the direction-change rollers 11 and 12 
which, together with backing rollers 13, are mounted on a support 30 which 
is pivotally connected to a pair of arms 31 and 32 held in the stationary 
sleeves 33 and 34 by setscrews 35 and 36 which allow the distance between 
the substrate and the lower pass of the belt to be adjusted and also allow 
the angle of inclination of the belt to be adjusted as represented by the 
arrow 40. 
The thickness of the cladding layer will thus depend on the position of 
this belt. 
The stainless steel belt is cooled on its internal surface by the direct 
spray of water from nozzles 41 and by circulating cooling water through 
the rollers 13. 
The substrate, along its inclined stretch, is supported on an array of 
rollers 42 which are mounted on a common frame 43 pivoted at 44 to the 
machine stand so that the angle of tilt can be correspondingly adjusted as 
represented by the arrow 45. 
At least the lower group of these rollers can be cooled by water as well. 
Forced cooling is also effected by blowing air unto the substrate, eg. via 
nozzles 14 and/or by spraying water onto the reverse side of the substrate 
from nozzles 15. The cooling water, collected by means of a stripper 16 
and a collecting tank 17, can be cooled in a heat exchanger and 
recirculated to the spray nozzles. 
In operation, the molten metal is pressed against the substrate by the 
endless belt 10 and congeals to form the layer 50 which can pass through 
quality control stations before the strip is cut into sheets of desired 
size. 
A pair of shoes 51 flank the layer 50 as it cools and define edge zones s 
free from the cladding.