Method and apparatus for producing strip products by a spray forming technique

A method of producing continuous lengths of metallic strip comprises the steps of forming a spray of molten metal particles, causing the particles to be deposited onto a surface of a hollow receptor roll, rotating and internally heating the receptor roll to form continuous lengths of metallic strip, holding the deposited particles in contact with the receptor roll surface by means of a roll positioned immediately upstream of the knife in the direction of rotation of the receptor roll, and peeling from the receptor roll surface such continuous lengths of metallic strip by means of a knife positioned adjacent to or in contact with the receptor roll surface. Apparatus for performing this process is also disclosed.

BACKGROUND OF THE INVENTION 
This invention relates to apparatus for and a continuous method of 
producing strip or sheet (hereinafter referred to simply as "strip") by a 
spray forming technique in which a plume or spray of metallic particles or 
droplets at elevated temperatures is deposited onto a suitably shaped 
receptor surface and removed therefrom in strip form. 
In one particular method of spray forming, a stream of molten metal falling 
freely under gravity is atomised by a system of high pressure jets to form 
the required plume or spray of droplets which, when they impinge on the 
suitably shaped receptor under appropriate conditions build up to form a 
solid artefact for subsequent hot compaction as required. 
Hitherto, it has been possible to produce sheets or plates of relatively 
thick section by such methods: however the correct conditions for the 
continuous production of near-to-final thickness strip have not been 
established. 
Spray forming techniques can be used to produce a wide variety of metallic 
strips of different compositions. Such techniques do, however, have 
particular application to the continuous production of electrotechnical 
steels. 
As is well known, the operation of transformers, motors, generators and 
like electrical machines depends upon the phenomenon of electromagnetic 
induction whereby current changes occurring in a (primary) coil are linked 
magnetically with a proximate (secondary) coil to cause a corresponding 
voltage to develop across the secondary winding, the value of which 
depends on the ratio of primary to secondary turns. The magnetic linkage 
effect is multiplied by many orders if the windings are formed upon a 
circuit of ferrous material so greatly enhancing the efficiency of the 
machine. As the current in the primary coil changes to establish a 
magnetic flux in the core, small currents called eddy or Foucault currents 
flow in the core material itself in a plane normal to the direction of the 
magnetic flux established in the core. Thus, if primary coil current is 
changing at, say, mains frequency (50 Hz), these eddy currents cause 
heating of the core material which is electrically conducting. Such 
heating effects are related in magnitude to the second power of the 
exciting frequency and represent power lost to the machine system. 
Therefore every effort is made to reduce the eddy currents. Two approaches 
are employed viz 
(a) make up the core from laminated sheet, each layer being typically 0.30 
mm thick and both surfaces carrying a very thin (micron) layer of 
electrical insulant; 
(b) increase the intrinsic resistivity of the material itself. 
We are aware, of course, that total power loss in a machine system has a 
second component, the hysteresis loss, and that this is dealt with by 
developing the grain structure of the core material so that in grain size 
and orientation magnetic domains of favourable form are engendered within 
each grain and thus improve the flux carrying capacity of the material 
with least loss of energy manifesting itself as wasteful heat. 
To those skilled in the art, it has long been known that the addition of 
silicon and aluminium to iron produces a wide range of electrotechnical 
steel strip incorporating these features and from which machine cores can 
be assembled. However, even the best known conventional steelmaking 
techniques are limited in the amount of material such as silicon and/or 
aluminium which can be added to iron to increase the electrical specific 
resistance of the ensuing material or improve the grain structure and 
hence domain dynamics because the addition of such elements causes the 
material to become so brittle as to prevent working due to a coarse grain 
formation. The presence in the material of silicon/aluminium in quantities 
of above about 3.25% means that the material cannot be cold reduced 
without the onset of cracking. Cold reduction is required for economic 
production and also to develop desired properties in important grades of 
electrotechnical steels as well as to improve surface quality. 
Attempts have been made to overcome these problems by the production of 
amorphous (non-crystalline) material by melt spinning processes but these 
materials are, to date, very thin as spun (about 40 micron max) and 
extremely brittle rendering them inappropriate for assembly into very 
large electrical machines. 
Therefore, the application of the spray forming technique to the continuous 
production of electrotechnical steel strip presents itself as a possible 
means of producing material comparable in mechanical handling to 
conventional material but with enhanced resistivity and/or grain/domain 
structure due to the fact that the spray forming technique permits the 
addition of silicon/aluminium to levels far beyond those possible with 
conventional techniques while still retaining a small grain size or 
permits the formation of alloys not possible by conventional processing 
due to, for example, segregation. 
It is an object of the present invention to provide apparatus for and 
methods of producing strip continuously by a strip forming process which 
at least alleviates disadvantages present in previous techniques. More 
especially, but not exclusively, it is an object of this invention to 
provide apparatus and methods by which electrotechnical steel strip of 
enhanced magnetic performance can be produced continuously by spray 
forming. 
The invention also sets out to demonstrate other benefits which accrue from 
the use of the technique to be described such as re-use of scrap material, 
use of compositions (alloys) with interesting magnetic properties hitherto 
prohibited by phase diagram limitations etc. 
SUMMARY OF THE INVENTION 
According to the present invention in one aspect there is provided 
apparatus for producing strip which comprises atomising means for 
producing a spray of metallic particles or droplets at elevated 
temperatures, a hollow receptor roll positioned below the atomising means 
on which the metallic particles or droplets are received and heating means 
positioned within the interior of the receptor roll and operable to vary 
in a controlled manner the temperature of the external surface of the 
receptor roll on which the metallic particles or droplets are deposited. 
The heating means preferably comprises furnace elements positioned within 
the body of the receptor roll The receptor roll may be produced from cast 
iron or mild steel and its wall thickness is selected to provide 
dimensional stability whilst minimising thermal mass to achieve rapid 
thermal response to temperature variations required during use. Typically, 
the wall thickness of the receptor wall lies in the range 0.64 to 3.18 cm. 
The receptor roll may be supported on a bearing cantiIevered from one side 
of a supporting framework, the framework supporting from its opposite side 
a cantilevered support for the furnace elements. 
The atomising means, receptor roll, furnace elements and framework are 
preferably enclosed within a gas-tight chamber. The chamber may be 
provided with gas tight glands to permit the entry of drive shafts for the 
receptor roll. 
The atomising means may comprise an array of nozzles connected to direct a 
plurality of gas jets onto the outer circumference of a stream of molten 
metal falling freely under gravity or pushed from an outlet nozzle of a 
crucible or the like. The crucible may be housed within an induction or 
resistance furnace positioned above the atomising means. 
A rigid knife with its blade in contact with the surface of the receptor 
roll may be provided to remove deposited material in strip form from the 
surface of the receptor roll. The angle of attack of the blade of the 
knife relative to the vertical may lie within the range 25.degree. to 
60.degree.. The knife may be positioned a short distance above the 
horizontal diameter of the receptor roll, this distance typically being 
between 2.54 and 10.16 cm for a roll of approximately 22.86 cm in outside 
diameter. A weighted roll with its axis generally parallel to that of the 
receptor roll may be positioned immediately above the knife to hold the 
deposited material onto the surface of the receptor roll before it is 
acted upon by the knife. For a receptor roll of approximately 22.86 cm 
outside diameter, the weighted roll is typically between 2.54 and 12.7 cm 
diameter. 
Strip material leaving the receptor wall may be fed continuously to an 
induction furnace for reheating to a temperature (typically between 
900.degree. and 1300.degree. C.). On leaving the reheat furnace, the strip 
may be compacted to the required density between compaction rolls. 
In another aspect, the invention provides a method of producing continuous 
lengths of metallic strip in which a spray of molten metal particles are 
deposited onto the surface of an internally heated hollow receptor roll 
and peeled from the receptor roll surface by means of a knife positioned 
adjacent to or in contact with the receptor roll surface, the deposited 
material being held in contact with the roll surface by means of a roll 
positioned immediately upstream of the knife in the direction of rotation 
of the roll. 
The spray of molten metal particles is preferably produced by directing a 
plurality of jets of gas onto a stream of molten metal falling freely 
under gravity or pushed from a tundish, crucible or the like. The 
atomising gas may comprise helium, argon or the like. The molten metal may 
be superheated prior to teeming. 
At the start of the process, a metallic Judas strip may be positioned on 
the surface of the receptor roll to receive the initially atomised 
material. The surface of the Judas strip may be toughened, e.g. by shot 
blasting. The Judas strip and sprayed material deposited thereon is 
subsequently pulled over the surface of the receptor roll to be removed 
therefrom. Alternatively, or additionally, the surface of the receptor 
roll (or a part thereof) is coated with a dried layer of, for example, 
colloidal silica after shot blasting. Rotation of the receptor roll is 
delayed for a short period of time at the start of the atomisation process 
to enable an initial build up of deposited material. 
In a further aspect, the invention provides strip material produced by the 
method exemplified in the preceding paragraphs.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
For the purposes of this particular description, reference will be made to 
the production of non-oriented electrical steel which, by conventional 
processing routes, is typically distinguished by having a combined 
silicon-aluminium content of between 3 and 3.25% by weight. It will be 
appreciated, however, that the apparatus illustrated can be employed to 
produce a wide range of other materials with higher levels of alloying 
constituents. 
The apparatus illustrated in FIG. 1 includes an induction or resistance 
furnace 1, housing a crucible 2 in which a melt of a carbon free 
electrical steel scrap or virgin melt material is produced. The furnace is 
constructed end insulated to avoid carbon pick-up in the melt. The 
crucible 2 is fitted with an hollow stopper rod 3 suitably equipped to 
carry a thermocouple so positioned within the stopper rod that its 
junction is sited at the centre of the melt to ensure that the temperature 
of the mass of the melt is accurately recorded. The stopper rod 3 and 
thermocouple can be selectively raised, preferably electromagnetically, to 
permit egress of melted metal through an orifice formed in the bottom of 
the crucible 2. 
The whole of the furnace assembly is contained within a stainless steel 
cheer 4 fitted with gas tight upper and lower water cooled jackets. To 
assist the egress of molten metal the chamber is supplied with a 
controllable gas pressure. A pressure of 0.2 to 0,6 bar is typical. The 
melting chamber is lined internally with a non-carbonaceous heat insulant 
material. The lower water jacket is fitted with a centrally disposed plug 
5 in which seats a ceramic cone nozzle capable of sustaining very high 
temperatures (e.g. a nozzle produced from boron nitride) and connected to 
the outlet of the crucible 2 by a heated refractory guide tube of 
typically 2.5 to 5 mm internal diameter. The plug 5 carries an array of 
finely machined and shaped orifices from which gas under pressure is 
directed onto the stream of molten metal emerging from the crucible 2 to 
produce a conical spray of atomised metal 6 of typically 30.degree. or 
15.degree. included half angle. The atomising jet orifices are connected 
to be supplied with dry impurity-free non-oxidising atomisation gas; 
bottled helium or argon are suitable gases although various types of 
electrotechnical steel processing annealing gas properly dried, clean and 
compressed would also be suitable. 
Situated at a suitable distance (typically 15.24 cm) below the orifice is 
located the surface of an internally heated receptor roll 10 whose outside 
diameter is typically of the order of 22.86 cm. The axis of the receptor 
roll 10 is horizontal and the roll is supported in a frame which also 
carries a rigid `knife` 11, the blade of which lies in contact with or 
closely adjacent to the surface of the receptor roll 10 and its function 
is to peel off metal deposited onto the surface of the receptor roll 10 in 
the form of a continuous strip. To assist the peeling action, a weighted 
small diameter roll 12, typically of 5.08 cm in diameter and with its axis 
parallel to the axis of the receptor roll, 10, is sited immediately above 
the knife blade 11 so that emergent peeled material is held down on to the 
peeling edge of the knife. 
Apart from assisting the peeling action of the knife, the roll 12 holds the 
emergent material in close contact with the surface of the receptor roll 
10 to prevent the material from peeling backwards to the point of 
deposition on the receptor roll 10 and thereby causing a discontinuity or 
other damage to the emergent material. The positioning of the knife 11 
relative to the horizontal diameter of the roll 10 and the angle of the 
peeling edge of the knife 11 are important. Typically, the knife is set to 
give a peeling line 3.81 cm above the horizontal diameter of the receptor 
roll and the included angle of the knife blade is from 25.degree. to 
60.degree.. The angle of attack of the knife blade relative to the 
vertical is typically from 3.degree. to 5.degree.. 
The receptor roll 10 is hollow so as to be of low thermal mass and 
therefore able to respond quickly to heat input supplied from furnace 
elements 14 contained in the body of the roll. While it is possible to use 
a mild steel receptor roll 10, best results have been achieved with a cast 
iron roll. The thickness of the wall of the hollow receptor roll 10 is 
typically approximately 1.5 cm. This affords strength and dimensional 
stability to the roll while keeping the thermal mass low to achieve rapid 
thermal response to heating and cooling as may be required during 
processing. To achieve this end, one bearing cantilevered from one side of 
the framework carries the receptor roll 10 shell while the opposite side 
of the framework carries the cantilevered support and wiring of the 
furnace which is also fitted with a thermocouple. The purpose of the 
furnace elements 14 is to heat the surface of the receptor roll 10 to 
provide for a degree of control of the metal deposition conditions. 
The whole of the receptor roll 10, furnace elements 14, framework etc is 
enclosed in shielding and the entire assembly from the water cooling 
bulkhead below the melting furnace is also contained in a second gas-tight 
stainless steel chamber 15 fitted with a cyclone and receiver to remove 
overspray arising from the deposition process. 
Penetrating the walls of this lower chamber are gas-tight glands which 
permit electric drive shafts to enter the chamber and rotate the receptor 
roll 10 so as to make possible a continuous process of spray deposition 
and peeling off from the surface of the receptor roll 10 of the contents 
of the upper furnace crucible. 
The peeled material is guided into an orifice in the wall of the deposition 
chamber and thence into a flat bed high energy input induction furnace 16 
of short length to reheat the peeled material prior to entry into yet 
another stainless steel sealed vessel inside which is a set of driven 
compaction rolls 17. The purpose of the rolls 17 is to ensure 100% density 
of the emergent material which is then coiled. Also, provision is made 
within this second chamber for a receptacle to catch the emergent stream 
of metal before atomisation is established. As soon as atomisation of the 
metal stream is established, this vessel can, by means of an arm rotating 
through a gland, be swung out of the way of the atomised stream. 
In operation of the apparatus described a furnace charge of carbon free 
electrolytic iron and ferro-silicon is prepared so that the ensuing 
composition will be typically 4.2% silicon by weight with the balance 
iron. 
The melting furnace is charged and all the equipment chambers are sealed 
and evacuated to remove air. The entire apparatus is flushed with argon 
(or other appropriate gas). The furnace is programmed to give a suitable 
melting regime and, by means of the thermocouple inside the stopper rod 3, 
the progress of the melt can be monitored. When the melting point of the 
charge has been reached (about 1500.degree. C.), the heating process is 
continued until the melt has attained about 1650.degree. C. or 
alternatively, about 150.degree. C.-200.degree. C. superheat. The purpose 
of the superheat is to keep the metal liquid until it emerges from the 
lower nozzle-into the atomising gas steam. Controlled delivery of the 
metal from the furnace 1 is further assisted by the establishment of a 
positive gas pressure in the upper chamber of about 0.3-0.5 bar. 
Once a stream of metal is teeming freely into a catcher device positioned 
below the crucible nozzle. the atomising gas is turned on at about 17 bar 
whereupon the spray jet is established and the catcher device swung out of 
the way. 
At this stage the spray does not fall upon the receptor roll 10 but upon an 
initially stationary lead strip of annealed (to render it pliable) thin 
gauge electrotechnical steel strip which has been shot blasted with grit 
and fed through the entire apparatus. The drives are set in motion and the 
lead strip now being covered with a spray deposited layer of metal is 
drawn through the system. When the end of the lead strip passes over the 
receptor roll 10, the surface of which has also been shot blasted, the 
spray impinges directly on the surface of the receptor roll and the 
material thus formed is pulled through the system. The receptor roll 
surface is heated by the receptor roll furnace to maintain the deposited 
material at about 600.degree. C. to prevent chilling. Provision is made to 
lift the compaction rolls to permit the lead strip to pass and then the 
roll gap is reduced to the desired gauge. On being removed from the roll 
10 by the knife 11 and cracker roll 12, the as-sprayed strip is re-heated 
to about 900.degree.-1000.degree. C. in the flat bed induction furnace 16 
and thereby rendered soft enough to compact to provide material with 
two-good surfaces after passage through the compaction rolls 17. It has 
been found that the use of a lead strip can be avoided by providing shaped 
plain surfaced sheet metal guides to carry the as-sprayed material into 
the re-heat furnace and thence to the compaction rolls and to the coiler 
grip. An alternative arrangement can also be used whereby the guides 
consist of broad linked chain. 
It has also been found that the process can be initiated without the use of 
a lead strip by coating the receptor roll 10 with a dried layer of 
colloidal silica after shot blasting. Before deposition starts the 
receptor roll is given a half revolution to present a hot section of 
surface to the jet and then letting the atomised stream dwell on the 
receptor roll 10 for about 4 seconds at the start of atomisation before 
causing the receptor roll 10 to rotate. 
The short period of static desposition provides a thickened initial edge 
for the knife 11 to plough under and prise away from the surface of the 
receptor roll 10. 
The speed of rotation of the receptor roll 10, guide and compaction rolls 
and coiler determines the as-sprayed thickness of the strip which is 
related to the final thickness by about 50% reduction and, typically, a 
through speed of about 2.54 cm per second produces an as-sprayed thickness 
of 2-3 mm using a 2.5 mm guide tube and the abovementioned pushing and 
atomisation pressures. 
The width of the strip deposited under these conditions varies with the 
rate of metal delivery, size of guide tube, included angle of spray cone 
or plume and whether or not a scanning atomiser has been employed to 
achieve flatter strip of 15.24 cm width or greater. 
As the metal droplets impinge on the receptor roll a porous layer is 
contructed but as further droplets impinge, a "Splatting" action is 
introduced whereby the semi-liquid droplets virtually merge with one 
another but cool so rapidly that no time is accorded for large grains to 
form such as would be the case if strip of the same composition had been 
prepared by conventional processes. Also, due to the fact that atomisation 
produces a droplet spread with a mean size of between 50 and 200 micron 
(preferably about 80 to 100 micron), this is small enough to permit the 
final strip to be cold rolled to produce good surfaces and exact gauges or 
merely cold skin-passed if desired. A porous layer is also formed on the 
upper surface due to entrainment of atomising gas and, since the upper 
surface is not in contact with any substrate, the surface texture is very 
rough. However, all these defects are removed by hot compaction. 
Time of flight and velocity of the various particle sizes contained in the 
spray or plume are influenced by the pushing and atomising pressures, the 
size of the guide tube or exit nozzle, the nature of the atomising gas, 
whether or not scanning is employed and the magnitude of the separation 
distance between emergent jet and receptor surface. It should be noted 
that the atomising gas velocity is very high and it is necessary that the 
diameter of the vessel in which desposition is proceeding is large enough 
to avoid turbulence or buffeting which will disturb the spray or plume and 
produce malformed material. The high speed gas also causes very rapid 
cooling and therefore the quantities of gas used should be as small as 
possible. 
In brief, for a system to produce strip continuously, what is required is a 
means of causing a receptor surface to intersect a coned plume of atomised 
particles at a suitable distance from the plume source, for this surface 
to be continuously renewed, maintained at a suitable temperature and for 
the deposited material to be removed and compacted. The procedures 
described above fulfil these requirements and strip may thus be produced 
continuously. 
The receptor roll 10 is illustrated in greater detail in FIGS. 2 and 3 of 
the drawings. As shown, the outer shell 21 of the roll houses an 
electrical heater 22 comprising a plurality of heating tubes 23 which span 
across the full width of the roll 10. The tubes 23 are supplied with 
electricity by means of a cable 24. The roll 10 is mounted on a stand 25 
and is driven through a drive sprocket 26. The relative positions of the 
peeling knife 11 and weighted roll 12 are shown more clearly in FIG. 3. As 
will be seen from this Figure, the knife 11 is mounted in a holder 27 
positioned to one side of the roll 10 and the weighted roller 12 is 
carried by a resiliently mounted arm.28 positioned above the roll 10. As 
will be seen from FIG. 4, the included angle of the blade of the knife 11 
may vary between 25.degree. and 60.degree.. Other variables include the 
actual point of peeling, the peeled strip width and thickness, the strip 
tension, the strip temperature and composition, and the roll diameter. 
From the foregoing it will be readily appreciated that this system can be 
employed to produce alloys with desirable magnetic and other properties of 
the type 13-16% aluminium balance iron which would not be possible to roll 
or work commercially unless by the method of continuous strip production 
by spray forming. 
It will be appreciated that the foregoing is exemplary of methods and 
apparatus in accordance with the inventions and that modifications can 
readily be made thereto without departing from the true scope of the 
invention.