Note: Descriptions are shown in the official language in which they were submitted.
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DESCRIPTION
~ APPARATUS AND METHOD~FOR~
CHILL CASTING OF METAL STRIP EMPLOYING A
CHROMIUM~CHILL SURFACE
BACKGROUND OF THE INVENTION AND THE PRIOR~ART
Methods are known;for making~continuou's~metal
strip, of crystalline as~uell~;~as amorphou~(gl~ass~y~
structure, directly from the melt~by~ depositing~molten~
metal onto the surface of a rapidly~mov~ing~chlll~body
whereon it is~quenched~to the~ solid~s~tate.~The~chill~
body may be a rotating~wheel,~cup~or cycl~inder,~'and~the~
molten metal may be deposited onto the~peripheral
surface of the wheel or onto the inn~er~-surface' of the~
cup or cyclinder.~; The~ch~ body may~a1so~be~a~
traveling belt~,~ usually~an;endless bel~t.~The~metal~may~
be deposited`onto the~surface~of~`the chill~body~by~
methods such as jetting~the molt~en~metal~ônt~o~'the~
surface, moving the surface in contact~with~a meniscu~s
or`pendant drop of the molten;~metal to~drag~out~a~film
of the metal, or by dipping the surfacé of~the; ch
body into a-bath of molten metal.
The surface of ~the chill body~must~meet;~
several requirements. ;First, it must be~wetted~by;~the~
molten metal, or else forlnation of continuous ~str`ip~will
not take place. Second, it must be non-reactive~with~
the molten metal, that is~to say the~ 1ten;met~al~must~
not attack, and must not weld to the chill surface,~or
else the strip cannot be cleanly separated~therefrom.
Third, it must ~lave good thermal conductivity to permit
rapid removal of heat as is necessary to effect rapld
solidification of the snolten metal to permit ormation ;
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of the molten metal, to permit formation of the glassy
structure, and to optimize the properties of the
metallic glass strip, especially to optimize its mag-
netic properties. Lastly, it must have sufficient wear
resistance in continuous production of metal strip by
the above-described quench casting process. Wear
resistance is an extremely important aspect of chill
body performance. High heat conductivity metals
previously proposed to serve as chill body surface, such
as copper, beryllium copper or silver, do not have the
desired wear characteristics. Others, such as stainless
steel, which would be expected to have good wear
characteristics, fall short in other respects, such as
~ailure to provide sufficient wetting, or good thermal
conductivity.
SUMMARY OF THE INVEMTION
.. . . .
In accordance with the present invention there
is provided an improvement in the method for~making
metal strip directly from the melt by depositing the
molten metal onto the surface of a rapidly moving chill
body, which improvernent comprises depositing the molten
metal onto the surface of a chill body constructed of a
metal selected from the group consisting of copper,
silver, molybdenum and alloys thereof having a surface
coating of chromium of thickness of from about 0.002 to
about 0.15 millimeter, and having surface roughness of
about 0.25 to about 3 micrometers.
The present invention further provides an
improvement in the apparatus for making thin metal
sections, such as splats or strip, directly from the
melt by depositing the molten metal onto the surface of
a rapidly moving chill body, which apparatus includes a
chill body having a surface adapted to receive molten
metal to be deposited thereon for rapid quenching
together with means functionally connected with said
chill body for depositing molten metal onto its surface,
wherein the improvement comprises providing a chill
body constructed of a metal selected ~rom the group
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consisting of copper, silver, molybdenum and alloys
thereof, having a surface coating of chromium of thick-
ness of from about 0.002 to about 0.15
millimeter, and having surface roughness of about 0.25
S to about 3 micrometers.
By the chill body being constructed of copper,
silver, molybdenum or alloys thereof it is rneant that
these metals furnish the heat extracting member of the
chill body, or the "heat sink", which absorbs the heat
of the molten metal to effect rapid quenching thereof
to the solid state, desirably at rates in the order of
104 - 106 K/sec., or higher, as may be required for
formation of metallic glass bodies from glass-forming
alloy melts. This therefore does not mean that in the
lS construction of the chill body other metals may not
be employed, e.g. for structural purposes, such as
reinforcement, and chill bodies incorporating other
metals are intended to be within the scope of the
appended claims.
I have surprisingly found that the above-
described chromium surface coat, of the specified thick-
ness and critically defined surface texture (surface
roughness~ is readily wetted by the molten metal,
especially by iron, nickel and/or cobalt-based alloys
which upon rapid quenching from the melt form amorphous
structures (metallic glasses). Particularly good
wetting seems to be obtained with glass-forming,
iron-based, boron-containing metal melts wherein the
metal component predominantly comprises iron. These are
of considerable practical interest because of their
outstanding soft magnetic properties, which make them
eminently suitable for use in electromagnetic induction
devices. The particular chromium surface further
provides for good adhesion of the solidified metal
strip, which is essential to ef~ect thorough quenching
of the metal if a ductile, amorphous metal strip is
desired, yet it also affords clean release of the
solidified strip from the surface. It is believed that
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the surprising improvement in so~t magnetic properties
of iron-based, boron-containing metallic glasses
quenched on the chill body of the present invention,
which has been observed, is due to such good adhesion
and thorough quenching.
Lastly, the particular chromium-coated chill
bodies of the present invention combine the above-
described advantageous wetting and quenching properties
with excellent wear resistance.
The benefi~s of the above-described chill
bodies having a chromium surface of specific thickness
and surface structure in the process of making metal
strip directly from the metal by depositing the molten
metal onto the rapidly moving surface of a chill body
are obtained regardless of the configuration of the
chill body. That is to say, the chill body may be a
rapidly rotating drum of which the exterior surface is
used as the chill surface; it may be a rapidly rotating
cylinder whereof the inner surface ~urnishes the chill
surface, a moving belt, a cup-shaped structure, or any
other structure. Further, any means of depositing the
molten metal onto the chill surface, e.g. jetting,
flowing, dragging, dipping and others may be empIoyed,
without restriction.
For purposes o~ the present invention, a strip
is a slender body whose transverse dimensions are ~uch
less then its length. In that context, strips may be
bodies such as ribbons, sheets or wires, of regular or
irregular cross-section.
3 0 BRIEF DESCRIPTION OF THE DRAWINGS
The annexed drawings further illùstrate the
present invention.
Fig. 1 is a cross-sectional view of an annular
chill roll, the exterior surface of which is coated
3 5 with a chromium in accordance with the present inven-
tion;
Fig. 2 is a cross-sectional view o~ an annular
chill roll having a ring of chromium-coated molybdenum
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inserted in its surface.
Fig. 3 is a cross-sectional view of a cylin-
drical chill body having an inner chromium-coated chill
surface inclined with respect to the axis of rotation;
Fig. 4 is a side view in partial cross-section
showing means for jetting molten metal onto a rotating
chill roll, and a rotating chill roll provided with a
chromium-coated chill surface;
Fig. 5 is a somewhat simplified perspective
view of apparatus including means for depositing molten
metal onto a chill surface in the form of a moving end-
less belt having a chromium surface.
DETAILE~ DESCRIPTION OF THE INVENTION, OF THE PREFERRED
EMBODIMENTS, AND OF THE BEST MODE PRESENTLY CONTEMPLATED
FOR ITS PRACTICE
Chill casting processes for making metal
strip - polycrystalline as well as amorphous (glassy)
metal strip - by depositing molten metal onto the~
surface of a rapidly moving chill surface of a heat
extracting member (chill body) are well known. It has
now been found that for use in such chill casting
processes, chill bodies having heat extracting members
of silver, copper, molybdenum or alloys thereof, which
heat extracting members have a chromium-coated chill
surface af very specific construction, have very
desirable properties, especially for casting metallic
glass strips of iron-based boron-containing alloys.
The chromium coating must be at least about
0.002 millimeter thick or else it is of little or no
benefit, inter alia for the reason that it provides
insufficient wear resistance. On the other hand, it may
not be thicker than about 0.15 millimeters. I have
found that substantially thicker chromium coatings
result in insuficient quenching of the melt, and in
general deterioration in physical properties of glassy
metal strip cast thereon, especially loss in ductility
and of magnetic properties, e.g. reduced maximum
induction and permeability. Good results are obtained
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with chromium coatings having thickness of from about
0.01 to about 0.1 millimeter, more preferably of from
about 0.01 to about 0.075 millimeter.
The second critical element is the surface
texture of the chromium coat. It has been thought by
those skilled in the metallic glass casting art that it
~ould be difficult or impossible to cast metallic glass
strip by quench casting techniques on chromium surfaces,
principally because of their relatively low heat con-
ductivity. Not only that, it was further found thatsuch strip, when cast on a smooth chromium surface,
even a smooth, thin chromium~surface of thickness within
the range of that contemplated by the present invention,
fails to adequately wet the surface and to adhere to the
surface sufficiently to obtain good quenching of the
strip. Consequently, strip cast on smooth chromium
surfaces tends to be brittle, to have non-uniform
properties, and to lack good magnetic properties. Such
strip is commercially unacceptable.
I have now surprisingly found that thin
chromium coats having very specific surface texture
avoid the above-described shortcomings, and instead
provide for a high degree of heat transfer from the melt
deposited onto the chill surface to the underlying heat
sink to effect rapid solidification o the melt, and for
a high degree of adherence of the solidified strip to
the chill surface, to effect thorough and rapid
quenching to below the crystallization temperature of
the metal strip, thereby producing strip having good
magnetic and physical properties. In order to obtain
these desirable results, the surface roughness must be
from about 0.25 to about 3.0 micrometers. Insuf~icient
surface roughness below about 0.25 micrometers will
tend to result in the above-described shortcomings of a
smooth chromium surface. A higher degree of surface
roughness above about 3.0 micrometers, while generally
providing sufficient ~uench rate and adhesion, results
in strip having undesirably high roughness on the
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surface cast in contact with the chill surface. Good
results are obtained with chill surface texture
corresponding to surface roughness of rom 0.5 to about
2.0 micrometers, preferably of from about 0.6 to about
1.5 micrometers. The most preferred surface texture is
a "satin finish", within the above-stated ranges of
surface roughness, a satin finish being defined as a
surface texture without visibly discernible lay, that is
to say that surface roughness is the same measured in
any direction. However, a satin finish, while providing
the best results, is not absolutely necessary, and
acceptable results are also obtained under conditions of
discernible lay, such as where the finish runs
longitudinal or transverse of the casting direction, or
in any direction therebetween.
T~hen amorphous metal strips are made by jet-
ting molten glass forming alloy against the surface of
a rapidly rotating chill body as, e.g., described in
U.S Pat. 4,077,462 to Bedell et al., or in U.S. Pat.
3,856,074 to Kavesh, the surface of the chill body
becomes gradually eroded. A rough, uneven track is
developed around the periphery of the chill body surface
whereon casting of the strip takes place. Further
casting into the same track produces strip of
unacceptable quality, having a rough surace and ragged
edges. The problem of chill surface wear in these
processes is even more acute when casting takes place
under vacuum. The absence of an intervening gas layer
in vacuum casting allows a larger area of the chill
surface to be impacted and wetted by the molten jet.
~nother factor which leads to severe wear on
conventional chill surfaces is inclusion in the alloy
being cast of appreciable amounts of refractory metals,
e.gO, molybdenum, tungsten, chromium, hafnium, iridium,
niobium, osmium, platinum, rhenium, rhodium, ruthenium,
tantalum, thorium, vanadium, and zirconium. Hence, use
of the chill surface of the present invention is
particularly advantageous when casting under vacuum
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(say under absolute pressure of less than about 25 mm.
Hg), or when casting glass-forming alloys containing one
or mQre refractory metals, and espe~ially when casting
such alloys under vacuum.
I S The form of the chill body and the mode of the
! casting operation are not critical for purposes of the
I present invention. For example, casting may take place
against the peripheral surface of a ~rapidly rotating
drum by jetting molten metal against that surface, as
disclosed in the above-mentioned patents to Bedell et
al. and Kavesh~ Casting may take place against the
exterior surface of a rotating drum by drawing out the
metal from a meniscus formed at a slotted nozzle, as
described in U.S. Pat. 3,522,836 to Ring, or from a
pendant, unconfined drop of molten metal as described in
U.S. Pat. 3,896,203 to Maringer et al. Alternatively,
the peripheral surface of the rotating chill drum may be
dipped into a bath of molten metal as described in U.S.
Pat. 3,8~1,450 to Mobley et al., or the molten metal may
be deposited under pressure from a slotted nozzle onto
the chill surface, as described in U.S. Pat. 4,14?,571
to Narasimhan. Furthermore, the chill surface may be
furnished by the interior surface of a rotating
cylinder, as des~ribed in U.S. Pat. 3,881,540 to Ravesh
25 and U.S. Pat. 3,881,542 to Polk et al~, or as shown by
Pond and Maddin in Trans. Met. Soc. AIME, 245 (1969)
2,475-6. Also, casting may take place into the nip two
counter-rotating chill rolls, as for example described
in U.S. Pat. 3,881,541 to Bedell, and by H.C. Chen and
30 C.E. Miller, in Rev. Sci. Instrum. 41, 1237 ~1970).
Moreover, the chill surface may be furnished by the open
concave surface of a rapidly rotating cup as disclosed
in U.S. Pat 2,825,108 to Pond; or a moving belt,
desirably a moving endless belt, as described in u.s.
Pat. 2,886,866 to Wade. The advantages of the use of
the specific chromium-coated chill body o~ the
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present invention are obtained regardless of the
construction of the chill body, and rega~dless of the
means for depositing the molten metal onto the chill
surface, that is to say whether by jetting, dragging
from a meniscus or a pendant drop, forcing it through a
slotted nozzle located in close proximity to the chill
surface or by dipping into a bath of molten metal, or by
any other suitable means.
The chromium coating or plating is suitably
applied to the substrate of copper, silver, molybdenum
or alloys thereof by means of electroplating, using
conventional electroplating procedures, although other
the procedures may be employed, if desired. Methods of
chromium plating are well known, and generally involve
passing a d.c. current through a suitable plating bath,
e.g. one containing chromic acid together with a~
suitable "catalystn, typically sulfate ion~provided by
sulfuric acid, and using the surface to be plated as the
cathode. The chromium plating operation can~be
facilitated, and the adhesion of the chromium coating
can be improved by first applying to the surface to be
plated a thin (e.g. less than about 0.01 millimeter
thick) "strike" coat of nickel, as is conventlonal in
chromium plating operation.
The surface texture, i.e. the above-described
surface roughness, may be provided by treating;the
chill surface by suitable means before or after the
chromium plating operation, or both before and after the~
chromium plating operation, with suitable abrasive. For
ease of treatment, and to protect the integrity of the
chromium coat, surface treatment b~fore the chromium
plating operation is preferred. Surface texture can
suitably be obtained by abrading the chill surface with
a suitable abrasive, such as emery cloth and the like,
or by impinging a suitable finely divided hard powder
against the chill surface, or by similar means. A very
effective means involves "slurry honing", which involves
impinging a fluid stream containing finely divided
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suspended abrasive particles against the surface to be
roughened.
In any event, methods of plating and of pro-
viding the specific surface texture are well known and
are not part of the present invention.
When the chill body is to be made of
molybdenum, it can be made by procedures usually em-
ployed for fabrication of molybdenum, including
machining from solid stock, such as cast pieces, or
fabrication by known powder metallurgical methods. A
particularly desirable embodiment of the present
invention is a composite chill body, especially a chill
roll, made of copper provided with a hoop of molybdenum,
as illustrated in Figs. 1 and 2. The casting surface
provided by the molybdenum is chromium plated and has
the above-described surface texture. With reference to
Fig. l, chill roll 1 made of copper is mounted for
rotation on shaft 2. The exterior surface of chill roll
l is provided with a hoop of molybdenum 3. In Pig. 1,
the hoop of molybdenum covers the total peripheral
surface of the chill roll. The molybdenum hoop may be
affixed to the copper chill roll, e.g. by shrink
fitting. Alternatively, a molybdenum surface may be
provided by any other conventional surface~coating
method, as for example oxyacetylene spraying, a method
which involves feeding a molybdenum wire into the cone
of an oxygen/acetylene flame to melt the metal, and then
propelling the molten metal in droplet form against the
surface to be coated. Other suitable methods include
plasma arc spraying and conventional cladding
procedures.
Another embodiment of the present invention
utilizing a copper-beryllium chill body from the con-
ventional copper-beryllium alloys is also suit-
able.
Detailed design and construction of apparatusof the present invention is within the capability of any
competent worker skilled in the art.
2 3
The following Example further illustrates the
present invention and sets forth the best mode presently
contemplated for its practice.
Example and Comparative Test
Apparatus employed was similar to that depic-
ted in Fig. 4 employing a water-cooled copper chill roll
having a 0.025 millimeter thick satin finish chromium
coating with surface roughness of about .76 - .80 micro-
meters (about 30-34 ~ inch). The chill roll had a
diameter of 38.1 cm (15 inch), and it was rotated at a
speed to provide peripheral velocity of rom 914 to 1067
meter/min (3000-3500 ft/min). The nozzle for depositing
the molten metal had an orifice of 2.54 cm (1 inch)
length and 0.5 mm (20 mil) width. The gap between the
chill surface and the nozzle was about 0.25 mm (about 10
mil). Alloy of composition Fe81~13.5Si3.5C2 (
percent) was ejected through the noæzle into contact
with the rotating chill surface under pressure at the
rate of about 4.53 kg./min. (10 lbs/min). It
solidified on the surface of the chill roll into a strip
2.54 cm (1 inch) wide and about 0.038 cm (1.5 mil)
thick.
A comparative run employing a copper chill
roll without the above-described chromium coating, but
under otherwise identical conditions, was made.
Properties of the strip obtained in the Example and in
the comparative run are summarized in the Table, below.
It was noted that the chromium coated chill
roll did not show any discernible wear after 110 kg of
metal had been cast in 12 consècutive runs of about
equal size, each on the same "track". A plain copper
wheel would show considerable wear after a single run,
and the chill surface would require "dressing" after
each individual run to restore the chill surface to the
necessary degree of smoothness required to make strip of
acceotable surface characteristics.
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TABLE
Magnetic Properties of Cast Strip
(data shown for beginning and end of Run)
Comparative Run Example
As Cast
Coercivity Hc (Oe) 0.11/0.18 0.055/0.078
Remanence B(oj (k ~auss) 5.2/4.3 7.2/6.8
Saturation Induction
at 10 30e B(l)(k Gauss) 6.7/5.6 8.0/7.8
Permeability 10 3 29/16 100/70
Core Loss (W/lb)(l) 0.69/0.78 0.65/0.56
(2)
VA Demand (V /lb) 37.8/43.2 21.7/16.7
Field Annealed t3)
Coercivity Hc (Oe) 0.11/0.11 0.04/0.045
Remanence B(o)(k Gauss) 8.0/2.6 13.8/12.5
Sat. Induction B(l)(k Gauss) 12.8/8.0 15.0/14.6
Permeability MmaX x 10 75/15 333/218
Core Loss (W/lb) 0.11/0.32 0.079/0.072
VA Demand V/lb) 0.98/18.4 0.145/0.184
(1) Loss of watts/lb of core in transformer.
(2) Volts-Amperes in driving coil of transformer
required to bring core to operating magnetization.
(3) Obtained on a toroid annealed as follows:
Temperature 365C
Time at Temperature: 2 hrs.
Atmosphere: dry argon
Magnetizing Field: 10 Oe, circumferential
Cooling rate to 100: about 15c/min.
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As the data in the Table demonstrate, the
strip cast on the chromium plated surface having
specific surface texture has substantially improved
magnetic properties as compared to strip cast in
5 conventional manner on a copper chill surface. Dif-
ferences in magnetic properties from beginning to the
end of the run are substantially less in each instance.
Transformers made from strip cast on such chromium
plated chill roll will have substantially improved
efficiency, in that they will have greatly reduced core
losses, and greatly reduced VA demand.
Since various changes and modifications may
be made in the invention without departing ~rom the
scope and essential characteristics thereof/ it is
intended that all matter contained in the above
description shall be interpreted as illwstrative only,
the invention being limited only by the scope of the
appended claims.
