[0001] This invention relates to molds for the continuous, horizontal casting of metallic
materials and processes for using the molds.
[0002] Metals, e.g., steel, nickel, or iron, are formed into shaped articles through a variety
of techniques. The continuous casting of molten metal is a particularly advantageous
means to form metal into forms useful for further processing. In these continous casting
processes, the molten metal is passed through a mold which shapes the metal and in
which solidification commences. Generally, the continuous casting processes fall into
one of two classes, a vertical casting process or a horizontal casting process.
[0003] In a vertical casting process, the molten metal is poured, through the force of gravity,
into a mold. The formed stock then passes downward and is redirected by rollers to
a horizontal orientation for subsequent handling and processing. While vertical casting
processes find commercial application, noticeable drawbacks are the height at which
the casting operation must occur and the energy and equipment that must be used to
reorient the continuous stock from the mold.
[0004] The horizontal casting process offers an alternative. In this process, molten metal
in a tundish is passes through a lower portion of the tundish and into a horizontally-oriented
mold in which the solidification of the stock commences. The stock is withdrawn from
the mold, usually with an oscillating pulling to facilitate the movement of the stock
in the mold. Since the operation is horizontal, the capital requirements and operational
problems associated with the vertical casting processes are not incurred.
[0005] While the horizontal continuous casting of metals can provide advantages, other
considerations must be taken into account. For example, a substantially fluid-tight
seal must exist between the tundish and the mold to prevent undue leakage of the molten
metal. Further, the mold, which is normally a fluid (e.g., water) cooled mold composed
of a metal of high heat conductivity such as a copper-based metal, is at a substantially
lower temperature than the metal in the tundish. Consequently, not only must coefficients
of thermal expansion be accommodated, but also, other problems can occur. For instance,
molten metal may pass into any crevices between the tundish and mold and solidify,
and these solids can cause scoring of the surface of the stock and may even induce
structrual weaknesses in the stock. Also, chipping of the solid may occur and pass
into the stock being produced, resulting in damage and imperfections in the stock
as well as the mold.
[0006] In order to overcome these problems, the common practice has been to tightly fit
the mold with a break ring. The break ring typically fits in the interior of the mold
at its inlet end and forms a ring of smaller internal dimensions than the interior
perimeter of the mold. The break ring is formed of a low heat conductivity ceramic
having a relatively smooth surface. Generally, the break ring is composed of boron
nitride. The break ring must be carefully machined to close tolerances to fit the
dimensions of the mold.
[0007] The break ring serves to provide insulation between the tundish opening (or nozzle)
and the mold, and aids in defining where the skin solidification of the stock occurs.
Hence, the metal may be molten when it contacts the break ring and commences solidification
as it approaches or contacts the mold. Thus, problems with leakage of molten metal
into crevices with subsequent solidification can be virtually avoided. However, the
oscillating movement of the stock as it is pulled from the mold, together with the
smaller cross-section opening formed by the break ring and the large cross-section
opening of the mold can lead to the formation of what is termed as witness marks,
that is, marks coinciding with the pushing of solidified metal surface back into the
molten metal flowing from the break ring. Moreover, with start-up or shut-down of
the continuous casting apparatus, thermal gradients and differentials in expansion
can lead to the damage of the break ring and adversely affect the quality of the stock.
[0008] As mentioned above, break rings must be carefully machined to fit the mold. Because
of the tight tolerances that must be met in fitting the break ring to the mold and
the machining costs, molds are usually of limited configurations such as circular,
square or rectangular in cross section. Long, narrow molds, for instance, can result
in a bowing or breaking in use of the break ring and untoward seepage of molten metal.
Hence, the size and cross-sectional configuration of the stock are limited to, e.g.,
billets, or blooms, and significant energy must be expended to work the stock into
the sought shape such as an I-beam or plate material.
[0009] Accordingly, a need has existed to provide continuous horizontal casting processes
which provide acceptable quality metal stock without the difficulties attendant with
the machining and fitting of break rings into molds. It would be further desirable
to provide processes in which witness marks were reduced in size, if not eliminated,
to enhance the appearance and strength of the stock. Moreover, the ability to produce
near net shape stock by continuous horizontal casting would be particularly desirable.
[0010] It has now been found possible to provide moulds for the continuous horizontal casting
of ferrous metals which do not depend upon the fitting of a separate break ring into
a mould, yet are able to provide acceptable quality stock. By means of some aspects
of the present invention, the moulds can provide stock with reduced, and sometimes
substantially eliminated, witness mark effects. By means of other aspects of the present
invention, moulds can be readily provided that are suitable for the continuous horizontal
casting of near net shape stock which is of acceptable quality.
[0011] According to the present invention there is provided a mould suitable for the continuous
horizontal casting of metal to form shaped stock which comprises a mould body of heat
conductive metal which defines by its interior surface a mould cavity having a transverse
cross-sectional configuration approximating the cross-section of the shaped stock
and which is adapted to contact coolant for indirect heat exchange from the interior
surface of the mould, the mould also defining an end plate continuous with the interior
surface of the mould, and a thermal barrier layer integral with the mould of at least
that portion of the end plate of the mould that is adjacent the interior surface of
the mould, which thermal barrier layer is adapted to contact a nozzle of a tundish
to supply molten ferrous metal to the mould, the thermal barrier layer having sufficient
thickness to prevent the solidification of the ferrous metal adjacent to the nozzle.
[0012] The present invention also provides a process for the continuous horizontal casting
of metal using the mould of the present invention.
[0013] In accordance with the present invention, the mould comprises an interior surface,
an end surface adapted to contact a nozzle through which molten metal passes and on
at least that portion of the end surface adjacent to the interior surface, an integral
thermal barrier of sufficient thickness to maintain the metal first contacting the
thermal barrier substantially molten. The thermal barrier may comprise any suitably
applied ceramic thermal barrier coating that is able to withstand the high temperatures
associated with the molten metal and is not chemically attacked by the molten metal.
[0014] The coating may provide a cross-section substantially the same as that of the mould
and can thereby minimize the degree of formation of witness marks. Moreover, since
the coating is integral with the mould, the cross-section configuration of the mould
need not be limited to those which are operable with separate break rings.
[0015] The metal being formed may be ferrous metal, e.g., steel or iron, or may be non-ferrous
such as nickel. Generally, the processes using the mould of the present invention
are most advantageous when handling high temperature metals.
[0016] Continuous casting moulds are typically fabricated from a relatively high conductivity
metal such as a copper-based metal. The mould is cooled with cooling fluid such as
water to remove heat from the mould and provide the casting of the ferrous stock.
The mould has an internal cross-sectional configuration approaching that of the extruded
stock. Generally, the cross-section is approximately regular through the length of
the mould in the direction of the flow of the steel. One end of the mould is adapted
to abut the nozzle from the tundish. For the sake of convenience, this end will be
termed the end plate for the purposes herein. The end plate may extend in a plane
substantially perpendicular to the axis of the mould or may have another suitable
configuration for contact with the nozzle of the tundish. In any event, the contact
should be substantially fluid-tight in order to prevent undue leakage of the molten
metal between the nozzle and the mould.
[0017] At least that portion of the end plate adjacent to the interior surface of the mould
is provided with an integral thermal barrier. The thickness of the thermal barrier
layer is preferably at least about 0.025 cm (about 250 microns). The thermal barrier
layer may extend only on the end plate of the mould and define an edge which smoothly
extends from the interior surface of the mould. The end plate may be bevelled adjacent
to the interior surface of the mould.
[0018] The barrier layer may overlap the portion of the interior surface of the mould adjacent
the end plate. The barrier layer may be contoured to taper to the interior surface
of the mould.
[0019] The thermal barrier may be ceramic and often is comprised of a refractory oxide.
Preferably, the thermal barrier is substantially inert to the molten metal. Typical
refractory oxides include alumina, silica, zirconia (especially yttrium-stabilized
zirconia), magnesia, chromia and mixtures and compounds thereof. Other ceramics include,
for example, silicon nitride, zirconium nitride, titanium carbide, and titanium nitride.
[0020] The integral thermal barrier may be applied by any suitable technique that provides
adequate adherence to the mold including under the temperatures and stresses that
exist in the casting operation. Particularly useful techniques include the thermal
spray processes in which the powder to form the coating is contacted under temperature
and velocity with the surface to be coated. Exemplary of these processes are the plasma
torch coating, detonation gun, hypersonic combustion spray, and flame spray coating
processes.
[0021] Undercoats and/or gradient coatings may also find application in assisting to provide
the strength and thermal shock resistance required for the thermal barrier. Undercoats
include the MCrAlY-type undercoats in which M is at least one of cobalt, nickel and
iron. Often the undercoat is 0.0025 to 0.02 cm (25 to 200 microns) in thickness. This
class of undercoats provides desirable adherence to the mold, including the copper-based
materials used in fabricating the mold, and in the over-layer of ceramic material.
The ceramic material may be formed in different layers or may be formed as a continuous
gradient of differing compositions to mitigate the effects of differentials in thermal
expansion and thermal shock. As can be well appreciated, the materials of the molds,
such as copper and copper-based metals, are often characterized by relatively large
coefficients of thermal expansion wherein ceramics generally have much lower coefficients
of thermal expansion. By varying the composition of the overlay from one which contains
a portion of the ceramic and a portion of a material that has both good bonding and
a higher coefficient of thermal expansion at the interior to one which comprises substantially
of the ceramic at the exterior surface that contacts the molten metal, beneficial
properties can be obtained including resistance to cracking or chipping of the thermal
barrier. Clearly, any cracking or chipping can result in the molten metal solidifying
and adversely affecting the quality of the stock.
[0022] More generally the thermal barrier preferably comprises a graded or multiple layer
structure combining refractory oxide with varying portions of metal to enhance thermal
shock stability.
[0023] The mould preferably has a cross-section of near net shape of an ultimate stock.
The mould may have, for example, a cross-section of an I-beam or of a plate.
[0024] The present invention will now be further described with reference to and as illustrated
in the accompanying drawings, but is in no manner limited thereto.
[0025] In the accompanying drawings:
Figure 1 is a schematic, longitudinal cross-sectional depiction of a tundish having
a nozzle and a mould;
Figure 2 is a schematic, longitudinal cross-sectional depiction of the entry portion
of a horizontal casting mould with a break ring; and
Figures 3, 4, and 5 are schematic, longitudinal cross-section depictions of the entry
portion of three horizontal casting moulds in accordance with the present invention.
[0026] With respect to Figure 1, tundish 100 contains molten metal 102. The tundish is generally
filled in a batch operation with molten metal. The tundish may be heated externally
to ensure that the metal remains molten and is usually fabricated from refractory
such as zirconia brick. The tundish is provided with nozzle 104 through which the
molten metal passes. The nozzle is also usually fabricated from refractory such as
zirconia brick. Zirconia brick can be porous and fragile; however, it is not generally
considered to be suitable for the area immediately adjacent the mold. In a conventional,
continuous horizontal casting apparatus, the break ring would be positioned between
the mold and nozzle.
[0027] The mold 106 is shown in schematic cross-section. The mold cooling channels are not
shown. The heat transfer to the coolant occurs at the periphery of mold at the area
generally designated as 108. Positioned between nozzle 104 at the interior surface
110 of mold 106 is thermal barrier 112. The partially solidified stock 114 is withdrawn
from mold 106 and is passed to a series of rollers 116 (only one depicted), some of
which are driven to pull the stock from the mold and pass it to further processing.
The driven rollers are typically operated such that the stock oscillates in the mold.
Alternative methods of gripping the stock and moving are also used; e.g., jaws which
grab the stock and move it.
[0028] Figure 2 schematically depicts a portion of a mold using a conventional break ring.
With reference to the drawing, mold 200 shuts nozzle 202 of the tundish. Break ring
204 is positioned between the nozzle and the interior surface 206 of the mold. Molten
metal 208 passes through the nozzle and break ring and thereafter fills the cross-section
of the mold defined by the interior surface 206 of the mold. Solidification of the
surface of the molten metal (skin formation) usually occurs at a point on the downstream
face of the break ring and on the interior surface of the mold which is designated
as zone 210. The oscillation of the stock pushes the skin back into molten metal and
causes the irregularities referred to as witness marks.
[0029] Fig.3 depicts a portion of mold 300 which is in accordance with the present invention.
Mold 300 has interior surface 302 and end plate 304 which has placed thereon thermal
barrier 306. In this embodiment of the invention, the thermal barrier is exclusively
on the end plate and the thermal barrier smoothly meets interior surface 302 and extends
the cross-section of the mold.
[0030] The thickness of the thermal barrier is sufficient that the molten metal passing
between nozzle 308 and the abutting surface of the thermal barrier does not solidify.
Molten metal which does seep into the crevice can, however, solidify well within the
crevice without undue adverse effect. The portion of the thermal barrier contacting
the nozzle 308 can be machined to fit snugly against the nozzle or may be left in
a roughened state, especially when a ceramic cement is used to secure the thermal
barrier to the nozzle.
[0031] The differential in temperature between the nozzle and the mold may often be 500°C.
or more. Accordingly, the thickness of the thermal barrier must be sufficient to prevent
significant cooling of the contact region between the molten metal, the thermal barrier
and the nozzle. Often, the thickness of the thermal barrier is at least about 0.025
cm (abut 250 microns), say, at least about 0.05 cm (about 500 microns), and often
is in the range of about 0.07 to 0.15 or 0.20 cm (about 700 to 1500 or 2000 microns).
[0032] As can be seen in Figure 3, the thermal barrier extends over at least a portion of
the end plate of the mold. By integrally securing the thermal barrier to the end plate,
strength is provided to the thermal barrier to enable it to withstand the forces associated
with the flowing molten metal, as well as provide enhanced thermal shock resistance.
For instance, the thermal barrier provides thermal insulation not only in a radial
direction toward the molten metal flow, but also in an axial direction.
[0033] The solidification (formation of skin) of the molten metal can occur while the molten
metal is in contact with the internal surface 310 of the thermal barrier. This is
possible since the thermal barrier is integral with the mold and hence molten metal
cannot seep between the thermal barrier and the mold as is possible when using a break
ring. With a smooth contour between the thermal barrier and the internal surface of
the mold, the formation of witness marks due to the oscillating of the stock from
the mold generally does not occur with the same severity as those formed when using
a break ring.
[0034] Many of the materials that we applied as thermal barriers can readily be machined.
Since the machining can be done when the thermal barrier is integral with the mold,
it is much more easily conducted than the machining of a break ring. The machining
can also produce a very smooth surface (less than 1.27 x 10⁻⁴cm (50 microinches),
rms), which enhances the ability to provide a high quality stock with minimum imperfections.
[0035] Fig.4 illustrates another embodiment of the present invention in which the thermal
barrier 406 on mold 400 is thicker at its interior edge 410. The end plate, 404 of
the mold shown, is beveled as it approaches the interior surface 402 of the mold.
[0036] Fig.5 illustrates yet another embodiment of the present invention in which the thermal
barrier 506 not only extends over a portion of the end plate 504 of the mold 500,
but also extends radially inwardly from interior surface 502 of the mold. As depicted,
nozzle 508 abuts with the thermal barrier proximate to the flow of molten metal and
a portion of end plate 504.
[0037] The strength and integrity provided by the thermal barrier extending over the end
plate of the mold enhances the ability of the portion of the thermal barrier extending
into the cavity of the mold to withstand the stresses of the flowing molten metal
and reduce the risk of chipping.
[0038] The interior surface 510 of the thermal barrier is shown as being contoured and
tapering into the interior surface 502 of the mold. Since the thermal barrier is integral
with the interior surface 502, the deleterious effects of which could be caused by
molten metal seeping betwen the components do not occur. Moreover, the contour at
least partially replaces the void space characteristics of the break ring design of
Figure 2, and can result in attentuated witness marks.
[0039] As can be seen from the foregoing description, the thermal barrier may constitute
a wide variety of configurations provided that the thermal barrier extends over at
least that portion of the end plate of the mold which is adjacent the interior surface
of the mold. The end plate of the mold, for instance, may be tapered or otherwise
configured to provide more thermal barrier adjacent the interior surfce of the mold,
such as for example by bevelling, routing, providing, for example, indentations.
[0040] Since the thermal barrier is integral with the mold, the transverse cross-section
of the mould may be in virtually any shape limited by the ability to provide proper
cooling to the flow of molten metal in the mould and the ability to apply (and machine,
if necessary) the thermal barrier. Hence, near net shape horizontal castings can be
produced with substantial reduction in capital and operating costs required to reform
a conventional billet or bloom into the desired ultimate shape such as, for example,
I-beam, plates or sheets.
[0041] Moreover, since the thermal barrier can be applied by a coating technique, moulds
can be readily repaired. Further, since using thermal barrier materials also exhibit
abrasion resistance, by coating a portion of the interior surface of the mould, such
as in Figure 5, the useful life of the mould may be enhanced.
EXAMPLE
[0042] A copper mould for the horizontal casting of steel is coated in accordance with the
present invention on the end plate, or flange face, using an undercoat of Co-Ni-Cr-Al-Y
and an overcoat of yttria stabilized zirconia. The throat of the copper mould is about
10.16 cm (about 4" long with a cross-section of 13.34 cm x 17.78 cm (5-1/4" x 7")
with the corners having approximately a 0.32 cm (1/8") radius. The total flange face
is about 27.78 cm x 23.34 cm (about 10-15/16" x 9-3/16") (with the throat centered
in the flange face) and corners with about a 5.08 cm (2") radius. The periphery of
the flange face has a 1.11 cm (7/16") wide lip about 0.95 cm (about 3/8") deep to
contain a zirconia block used as a nozzle for pouring steel. Four separate moulds
as prepared. In all cases, the copper is first grit-blasted using 60 mesh alumina
grit and then plasma sprayed with an undercoat of nominally 32Ni-21Cr-8Al-0.5Y-Bal
Co., all in weight percent, to a thickness of 0.0076 to 0.0127 cm (0.003 to 0.005
inches). Over this metallic undercoat a zirconia coating consisting of ZrO₂-7 wt.%
Y₂O₃ is applied.
[0043] In one of the moulds the substrate is ground to a sharp corner prior to grit blasting
and undercoating and then oversprayed with a total thickness of 0.0508-0.0635 cm (0.020-0.025
inches) of zirconia. The edge of the zirconia coating at the interior surface of the
mould has a slight radius since some curvature is inherent in the coating process.
In another mould the copper at the flange face is ground to a radius of about 0.127
cm (about 0.050 inches) before grit blasting and undercoating and subsequently coated
at a 90° angle of impingement to a total coating thickness of about 1.308 cm (about
0.515 inches). A portion of this segment is then ground to a total zirconia coating
thickness of 0.061 cm (0.024 inches), still leaving a small radius of curvature on
the coated corner. In a third mould, the substrate is ground to a corner radius of
about 0.127 cm (about 0.050 inches) prior to coating and subsequently coated at an
angle of impingement slightly less than 90° to form a lip on the corner. The total
zirconia coating thickness is about 0.325 cm (about 0.128 inches). A small portion
of this segment is subsequently ground to a total coating thickness of 0.0577 cm (0.0227
inches) leaving a very sharp corner and a slightly negative angle of zirconia on the
corner. An advantage of this configuration may be a somewhat better bond at the corner.
In the fourth mould, the copper substrate is left with a sharp, round corner and about
0.307 cm (about 0.121 inches) of zirconia is applied. A portion of this segment is
subsequently ground to a thickness of 0.145 cm (0.057 inches) leaving a very sharp
corner on the zirconia. This remaining thickness of zirconia should be equivalent,
thermally, to that which is currently used on conventional boron nitride break rings.
The entire flange face could be ground to a uniform coating thickness.
[0044] The above-described moulds having a thermal barrier may be employed for the continuous
horizontal casting of metal such as, for example, steel.
1. A mould suitable for the continuous horizontal casting of metal to form shaped
stock which comprises a mould body of heat conductive metal which defines by its interior
surface a mould cavity having a transverse cross-sectional configuration approximating
the cross-section of the shaped stock and which is adapted to contact coolant for
indirect heat exchange from the interior surface of the mould, the mould also defining
an end plate continuous with the interior surface of the mould, and a thermal barrier
layer integral with the mould of at least that portion of the end plate of the mould
that is adjacent the interior surface of the mould, which thermal barrier layer is
adapted to contact a nozzle of a tundish to supply molten ferrous metal to the mould,
the thermal barrier layer having sufficient thickness to prevent the solidification
of the ferrous metal adjacent to the nozzle.
2. A mould according to claim 1, wherein the thickness of the thermal barrier layer
is at least 0.025 cm (about 250 microns).
3. A mould according to claim 1 or 2, wherein the thermal barrier comprises a refractory
oxide.
4. A mould according to claim 3, wherein the refractory oxide for the thermal barrier
layer comprises at least one of zirconia, yttrium-stabilized zirconia, magnesia, silica,
alumina and compounds thereof.
5. A mould according to claim 3 or 4, wherein an undercoat is provided between the
refractory oxide and the mould.
6. A mould according to claim 5, wherein the undercoat comprises MCrAlY, wherein M
is at least one of nickel, cobalt and iron.
7. A mould according to any of claims 3 to 6, wherein the thermal barrier comprises
a graded or multiple layer structure combining refractory oxide with varying portions
of metal to enhance thermal shock stability.
8. A mould according to any of claims 1 to 7, wherein the thermal barrier layer extends
only on the end plate of the mould and defines an edge which smoothly extends from
the interior surface of the mould.
9. A mould according to claim 8, wherein the end plate is bevelled adjacent to the
interior surface of the mould.
10. A mould according to any of claims 1 to 7, wherein the barrier layer overlaps
the portion of the interior surface of the mould adjacent the end plate.
11. A mould according to claim 10, wherein the barrier layer is contoured to taper
to the interior surface of the mould.
12. A mould according to any of claims 1 to 11, wherein the mould is comprised of
copper-based metal.
13. A mould according to any of claims 1 to 12, having a cross-section of a near net
shape of an ultimate stock.
14. A mould according to any of claims 1 to 13 having a cross-section of an I-beam
or of a plate.
15. A process for the continuous horizontal casting of metal using a mould according
to any of claims 1 to 14.