[0001] This invention relates to roll casting of molten metal. More particularly, this invention
relates to improvements in apparatus controlling the flow of molten metal from a reservoir
to a rolling mechanism.
[0002] The processing of molten metal by continuous casting to convert it to plate or sheet
fabricatable into various shapes conventionally involves the delivery of molten metal
to a pair of rollers from a casting nozzle comprising an elongated nozzle tip.
[0003] Process economics would justify the continuous casting and subsequent rolling of
wide sheets, i.e., over 40 inches in width, as well as faster rolling speeds, i.e.,
200 lb/in/hr. However, shortcomings in nozzle tip design resulting in nonuniform molten
metal temperature and exit velocities of the molten metal entering the nip of the
rollers have prevented use of such widths and speeds.
[0004] These problems in nozzle tip design, including nonuniform metal flow velocity profiles
across the nozzle tip and nonuniform temperature distribution, as well as flow disturbances
adjacent the side risers of the nozzle and any spacers which may be present within
the nozzle, can result in hot spots in roll caster and consequently cause bleed out
at high speed casting. Furthermore, flow disturbances and separation caused by the
internal structures of the nozzle tip can cause surface defects on the resulting cast
plate or sheet. The latter condition of flow disturbances is particularly complicated
by the necessity of utilizing some sort of spacers to support the top wall of the
nozzle and to maintain uniformity of spacing between the top wall and bottom wall
of the nozzle when attempting to cast wide plate or sheet by continuous casting techniques.
[0005] In the prior art, regulation of metal flow has been attempted using divergent channels
which may contain baffles. For example, Chateau et al U.S. Patent 4,153,101 provide
a nozzle having a lower plate and upper plate separated by cross pieces and side end
portions which are divergent along at least a portion adjacent the end of the nozzle.
[0006] Blossey et al U.S. Patent 3,799,410 show a feed tip having baffles therein which
coact in controlling the direction of flow of molten metal through the cavity in such
a manner said to insure continuous distribution of molten metal to the nozzle uniformly
throughout its length.
[0007] However, the control of the metal flow velocity as well as uniform temperature distribution
within the nozzle, particularly when a wide casting strip is desired, has been found
to involve design criteria which are not satisfied by the prior art.
[0008] According to the invention there is provided an improved roll caster tip apparatus
comprising:
(a) a molten metal reservoir comprising a bottom plate and at least one sidewall;
and
(b) a tip member attached to said sidewall comprising:
(i) a top wall;
(ii) a bottom wall; and
(iii) a pair of side riser members
between said top wall and said bottom wall, characterized by said side riser members
(44, 46) and said top wall (36) and bottom wall (40) forming a converging passageway
(50) terminating in an exit port (52) for molten metal to flow from said reservoir
(2) to a pair of rollers (60, 62), said side risers (44, 46) converging toward one
another at said exit port (52) by being spaced apart a shorter distance at said exit
port (52) than at said sidewall (16) whereby metal flow separation along said side
riser members (44, 46) will be mitigated, thereby providing uniform metal velocity
across the width of the tip member (32).
[0009] Also provided in accordance with the invention are further embodiments wherein a
stream-lined spacer member is included between top and bottom walls of the tip member;
the dimensions of the spacer member and top and bottom walls are preselected to provide
near Hele-Shaw flow conditions; and a baffle member is provided in the reservoir.
[0010] In the accompanying drawings:
Figure 1 is a side view in section of the apparatus of the invention.
Figure 2 is a top view in section of the apparatus of the invention.
Figure 3 is a top view in section of the spacer used in the apparatus of the invention.
Figure 4 is an end view in section of the apparatus shown in Figure 2 taken along
lines IV-IV.
Figure 5 is an end view in section of the apparatus shown in Figure 2 taken along
lines V-V.
Figure 6 is an end view in section of another embodiment of the view shown in Figure
5.
Figure 7 is a top view in section of another embodiment of the invention.
Figure 8 is a top view in section of yet another embodiment of the invention.
Figures 9-16 are graphs which respectively show the metal velocity profiles parallel
and perpendicular to the metal flow across a nozzle tip at two casting rates and at
two measurement positions with respect to the nozzle exit port.
[0011] Referring in particular to Figures 1 and 2, the apparatus of the invention includes
a reservoir generally indicated at 2 and a tip member attached thereto and generally
indicated at 32. Reservoir 2 comprises a bottom plate 6, a pair of end walls 10 and
12 and sidewalls 16 and 18. Mounted within reservoir 2 is a flow- restricting member
24 which forms an opening 26 to regulate the flow of molten metal 20 in reservoir
2 into tip member 32, as will be described in more detail below. The level of molten
metal 20 in reservoir 2 is maintained by metal level control 92 through which molten
metal flows via spout 96 from molten metal source 90. Metal level control 92 controls
the flow rate of metal into reservoir 2 using a float to determine and control the
level of molten metal in reservoir 2.
[0012] Tip member 32 serves to supply a flow of molten metal from reservoir 2 to the nip
of a pair of rollers 60 and 62, as shown in Figure 1. This flow of a ribbon of molten
metal should be of uniform velocity and temperature distribution across the entire
width of tip member 32 which may vary commercially from as little as 36 inches to
as much as 60 inches or more. Maintaining such uniform metal flow and temperature
characteristics for widths of 60 inches or more have been unattainable in the prior
art.
[0013] Tip member 32 comprises a top wall 36 and a bottom wall 40 supported by tip clamp
members 38 and 42. As shown in Figures 1, 2 and 4, top wall 36 and bottom wall 40
are joined together by side riser members 44 and 46 to define a passageway 50. Top
wall 36, bottom wall 40 and side riser members 44 and 46 are all joined, at one end,
to reservoir sidewall 16, as shown in Figure 4. An opening in sidewall 16 conforms
spatially to the passageway defined by the joining together of the wall members comprising
tip member 32 at their juncture with sidewall 16.
[0014] As shown in Figure 1, the facing surfaces of top wall 36 and bottom wall 40 are,
preferably, essentially parallel from the ends joined to sidewall 16 to exit port
52 at the nip of rollers 60 and 62. Thus, in the preferred embodiment, internal passageway
50 within tip member 32 is of uniform height. However, top wall 36 and bottom wall
40 may converge slightly, i.e., up to about 5°, to insure that there is no divergence.
When convergence of the top wall and bottom wall is used, reference herein to spacing
between the top wall and the bottom wall will mean average spacing distance.
[0015] Side riser members 44 and 46, however, are positioned to be convergent at the exit
port 52 of tip member 32. By making side riser members convergent, flow separation
and reverse flow will be eliminated near the side riser member. The convergent channel
formed thereby provides a favorable pressure gradient along the walls and an accelerating
main flow which limits the boundary layer thickness growth of the flowing metal downstream.
This eliminates, or at least reduces to a minimum, one cause of nonuniformity in the
metal flow velocity found in the prior art.
[0016] As shown in Figure 2, this convergence of the side riser members may be represented
by straight (i.e., linear) riser members which are mounted to slant toward one another
or converge. While the slanting or convergence of side risers 44 and 46 has been somewhat
exaggerated in Figure 2 for illustrative purposes, the convergence angle may be from
1 to 45°, preferably from 1 to 15°, and most preferably from 2 to 10°.
[0017] Alternatively, the side riser members may be curved in either a convex or concave
curvature. As shown in Figure 7, side riser members 44' and 46' are convex as viewed
from their inner, facing, surfaces, i.e., from passageway 50'. This provides a convergence
which tapers off in rate as the exit port 52 is approached.
[0018] In another embodiment, as shown in Figure 8, side riser members 44" and 46" are concave
as viewed from their inner, facing surfaces, i.e., from passageway 50". This provides
a convergence having an increasing rate as exit port 52 is approached.
[0019] As previously stated, one of the goals of the improved apparatus of the invention
is to permit the casting of very wide sheet, i.e., 60" or more, while maintaining
uniform metal flow and temperature conditions. To achieve this, the spacing between
top wall 36 and bottom wall 40 must be uniformly maintained across the width of passageway
50. This necessitates the use of one or more spacers to maintain the desired uniform
distance between top wall 36 and bottom wall 40 which may be as small as 0.194 inch
at exit port 52. The use of a spacer is not new; however, prior art spacers were not
necessarily designed or positioned to provide minimal interference with the desired
uniform metal flow characteristics. In Figure 3, a spacer 70 is illustrated which
has been designed to minimize adverse effects on flow conditions. The leading edge
72 of spacer 70, which faces the flow of metal, is curved to permit the metal flow
to smoothly pass on both sides. The trailing edges 74 and 76 of spacer 70 terminate
in a point 78 to provide a streamlined shape to minimize disturbance to the main flow
of metal and eliminate or minimize separation.
[0020] To achieve the desired streamline shape and flow characteristics, the width "d" of
spacer 70, measured at its widest point as shown in Figure 3, should not exceed 15%
of the chord length "L" of spacer 70.
[0021] The presence of one or more spacers in the metal flow path can affect the flow profile.
As shown in Figure 2, the positioning of spacers 70 with respect to their distance
from exit port 52 is.also important since a wake profile is developed denoting the
flow region behind a solid body, i.e., spacer 70, placed in the stream of molten metal.
The velocities of the metal flow in the wake are smaller than those in the main stream,
and the losses in the wake amount to a loss of momentum which is due to the drag on
the spacer. The spread of the wake increases as the distance from the spacer increases
and, therefore, the differences between the velocity in the wake and that outside
the wake become smaller as the distance from the spacer increases.
[0022] To recover at least about 95% of the velocity of the main stream in the wake area,
it is important to position spacer 70 a minimum distance from exit port 52. If spacer
70 is positioned from exit port 52 a distance at least one and one-half, preferably
two, and most preferably three or more, times the length of the chord of spacer 70
along the larger dimension, e.g., length "L" in Figure 3 extending from a point on
the spacer closest to the reservoir to the terminus of the streamline portion of the
spacer, the desired 95% recovery of velocity of metal flow will be achieved by the
time the metal reaches exit port 52. Uniformity of metal velocity may then be achieved
with minimum interference from spacers if they are used.
[0023] To achieve the desired uniform flow profile, it is preferable that the main stream
flow have a Hele-Shaw profile, i.e., a reduced Reynolds number of less than 1. However,
in practice, due to geometry constraints, it may not be possible to maintain the Reynolds
number below unity. It has been observed in experiments that a flow having a reduced
Reynolds number of 400 or less provides an acceptable uniformity of flow profile.
Preferably, however, the reduced Reynolds number is less than 200, and most preferably
the reduced Reynolds number is less than 1.
[0024] The criterion on which Helle-Shaw flow, or a nearly Hele-Shaw flow condition, takes
place is given by the reduced Reynolds number, R
*, in accordance with the following equation:
wherein: R* = not greater than 400, preferably less than 200, and most preferably less than 1;
U = average velocity of metal entering the tip in cm/sec.;
L = the chord length of the spacer;
u = kinematic viscosity of molten aluminum (approximately 5.17 x 10 -3 cm2/sec.); and
h = 1/2 the height between the top wall and the bottom wall.
[0025] The foregoing parameters insure the preservation of entry metal flow profiles within
nozzle tip member 32 which will deliver a band of molten metal to rollers 60 and 62
having a uniform velocity and temperature distribution to inhibit sticking and heat
transfer problems during initial rolling, if it is assumed that metal at a uniform
velocity is delivered to nozzle tip member 32 from reservoir 2. However, if the metal
flow into nozzle tip member 32 is non-uniform, it may be impossible to develop a uniform
metal flow velocity downstream because of the Hele-Shaw flow conditions which preserve
the velocity profile of the molten metal after its entry into the tip. In other words,
if the entrance velocity is nonuniform, the Hele-Shaw flow conditions will preserve
this nununi- formity as the metal flows through the tip. Thus, it is imperative that
the entrance velocity of the molten metal be as uniform as possible.
[0026] To provide for a uniform flow of metal into nozzle tip member 32, a baffle 24 is
placed in reservoir 2, as shown in Figures 1, 2, 4 and 5. Baffle 24, as shown in Figure
4, extends across the entire width of nozzle tip member 32 from side riser member
44 to side riser member 46. Baffle 24 extends down from the top of reservoir 2 below
the surface of the molten metal in the reservoir to a point just above bottom plate
6 of reservoir 2 to form a passageway 26 which extends across the entire width of
reservoir 2. As reservoir 2 is replenished with molten metal from molten metal source
90, baffle 24 provides a shielding from any turbulence created in reservoir 2 by such
additions and provides uniform friction across the entire width of nozzle 32. The
feeding of a steady, uniform flow of molten metal into nozzle tip member 32 is thereby
assured.
[0027] Figure 6 shows an alternate embodiment wherein baffle 24' is provided with a series
of holes 26' across the bottom portion of baffle 24'. The function of holes 26', which
are of uniform diameter, is to provide uniform friction across the entire width of
nozzle tip 32 at its jointure to wall 16 of reservoir 2 to insure uniform entrance
velocity of the molten metal into nozzle tip 32 in similar fashion to the function
of opening 26 created by the position of baffle member 24.
[0028] Figures 9 through 16 illustrate typical metal velocity profiles which can be expected
utilizing the teachings of the invention in a casting apparatus having a 68 inch wide
tip and using respective casting rates of 80 lbs/hr/in and 180 lbs/hr/in. In each
instance, a spacer having a 1-1/2 inch chord length was located 5 inches from the
exit port of the nozzle tip (measured from the trailing edge of the spacer). This
location of the spacer from the exit port was possible because the Hele-Shaw flow
conditions insure quicker recovery of the flat velocity profile downstream of the
spacers.
[0029] Figures 9, 10, 13 and 14 show measurements taken 7-1/2 inches from the exit port,
i.e., before the metal flow encounters the leading edge of the spacer, while Figures
11, 12, 15 and 16 represent measurements taken 2-1/2 inches from the exit port, i.e.,
2-1/2 inches beyond the trailing edge of the spacer. At both the 2-1/2 inch and 7-1/2
inch measurement points, the metal velocity was measured parallel to the metal flow
and perpendicular to the metal flow, i.e., toward the side risers. Hele-Shaw flow
conditions ensure quicker-recovery of the flat velocity profile downstream of the
spacers.
[0030] In each instance, a comparison measurement was also taken with a nozzle tip having
divergent side risers. Plots of the metal flow velocities in the nozzle tips having
divergent side risers are shown in solid lines, and the metal flow velocities in the
nozzle tips of the invention having convergent side risers are shown by the dotted
lines.
[0031] Thus, the invention provides an improved flow control of molten metal from a reservoir
to a rolling mechanism for the direct roll casting of metal plate or sheet from molten
metal. Uniform metal velocity and temperature control within the nozzle tip assures
the minimization of problems with sticking of metal to the rollers as well as heat
transfer problems which have characterized prior art approaches in the past.
1. An improved roll caster tip apparatus comprising:
(a) a molten metal reservoir (2) comprising a bottom plate (6) and at least one sidewall
(16,18); and
(b) a tip member (32) attached to said sidewall (16) comprising:
(i) a top wall (36);
(ii) a bottom wall (40); and
(iii) a pair of side riser members (44,46)
between said top wall (36) and said bottom wall (40), characterized by said side riser
members (44,46) and said top wall (36) and bottom wall (40) forming a converging passageway
(50) terminating in an exit port (52) for molten metal to flow from said reservoir
(2) to a pair of rollers (60,62), said side riser member (44,46) converging toward
one another at said exit port (52) by being spaced apart a shorter distance at said
exit port (52) than at said sidewall (16) whereby metal flow separation along said
side riser members (44,46) will be mitigated, thereby providing uniform metal velocity
across the width of the tip member (32).
2. An apparatus according to claim 1, characterized in that the convergence of said
side riser members (44,46) is from 1° to less than 45°, preferably from 1 to 15°,
and more preferably from 2 to 10°.
3. An apparatus according to claim 1 or 2, characterized in that said side riser members
(44,46) are curved to converge toward one another at the exit port (52) with respect
to the spacing apart of the side riser members (44,46) at said sidewall (16), whereby
preferably either said curved side riser members (44',46') have a convex curvature
with respect to the convergence of said side riser members whereby the rate of convergence
decreases as the molten metal approaches said exit port (52), or said curved side
riser members (44",46") have a concave curvature with respect to the convergence of
said side riser members whereby the rate of convergence increases as the molten metal
approaches said exit port (52).
4. An apparatus according to any one of claims 1 to 3, further characterized by having
at least one spacer member (70) between said top wall (36) and said bottom wall (40)
spaced from said side riser members (44,46) to provide support for said top wall (36)
and bottom wall (40), said spacer member (70) having a curved leading portion (72)
facing said reservoir (2) and a streamlined trailing portion (74¡76) extending in
the direction away from said reservoir (2), said streamlined portion (74,76) of said
spacer (70) preferably terminating in a point (78), whereby interference with metal
flow velocity by flow separation caused by said spacer (70) is minimized.
5. An apparatus according to claim 4, characterized in that the maximum width (d)
of said spacer (70) is not greater than 15% of the length (L) of the chord extending
from a point on said spacer (70) closest to said reservoir (2) to the terminus of
said streamlined portion (74,76) of said spacer (70).
6. An apparatus according to claim 4 or 5, characterized in that said spacer (70)
is located in said tip member (32) at a distance from said exit port (52) at least
two times (if desired, 3 or more times) the length (L) of said spacer (70), whereby
preferably:
(1) said streamlined portion (74,76) of said spacer (70) terminates in a point (78)
and said spacing of said exit port (52) from said spacer (70) is measured from said
point (78): and/or
(2) the length (L) of said spacer (70) is measured as a chord extending from a point
on said spacer (70) closest to said reservoir (2) to the terminus of said streamlined
portion (74,76) on said spacer (70).
7. An apparatus according to any one of the preceding claims, further characterized
by having at least one spacer member (70) between said top wall (36) and said bottom
wall (40) spaced from said side riser member (44,46) to provide support for said top
wall (36) and bottom wall (40) and having a chord length (L) extending from the leading
edge (72) of the spacer (70) facing said reservoir (2) to the trailing edge (74,76)
of said spacer (70) facing said exit port (52); said chord length (L) of said spacer
(70) and the distance between said top wall (36) and bottom wall (40) of said nozzle
tip (32) being preselected with respect to the velocity and viscosity of the molten
metal flowing through said tip (32) to provide at least near Hele-Shaw flow conditions
defined by a reduced Reynolds number of not more than 10.
8. An apparatus according to any one of the preceding claims, wherein said sidewall
(16) of said reservoir (2) has an opening extending along the entire width of said
nozzle tip member (32); and further characterized by a baffle member (24) in said
reservoir (2) extending across the entire width of said sidewall (16) opening and
providing a uniform friction for the metal flowing from said reservoir (2) into said
nozzle tip (32) whereby metal flowing into said tip member (32) from said reservoir
(2) has a uniform velocity across the entire width of said nozzle (32).
9. An apparatus according to claim 8, characterized in that said baffle member (24)
is spaced from said bottom plate (6) a sufficient distance to provide a metal flow
passageway (26) therebetween having a uniform height across the entire width of said
nozzle tip member (32) whereby said passageway (26) will provide a uniform friction
for said molten metal flowing into said nozzle tip member (32) to provide uniform
velocity of metal flow into said nozzle tip member (32).
10. An apparatus according to claim 8, characterized in that said baffle member (24')
extends downwardly in said reservoir (2) to said bottom plate (6) and a series of
uniformly sized openings (26') is provided in said baffle (24') across the entire
width of said nozzle tip member (32) whereby said openings (26') provide a uniform
friction for said molten metal flowing into said nozzle tip member (32) to provide
uniform velocity of metal flow into said nozzle tip member (32).