Direct Cast Strip Thickness Control

The invention relates to the production of metal sheet directly from the melt. In particular, it relates to casting of metal sheet on the outside surface of a chilled, cylindrical drum. A layer of melt is typically delivered to the casting surface by means of a tundish. The tundish is open at one end but is closely adjacent and mating with the casting surface on that end to deliver melt to the surface without leaking between the tundish and casting surface. The level of the melt, as well as other parameters of the casting run, affects the thickness of the cast strip. Before the present invention, the thickness tended to be greater at the edges due to a higher rate of heat extraction by the cooling drum at and near the edges of the strip relative to the center. Subsequent cold rolling of strip product generally requires the opposite contour, i.e., a strip which is slightly thicker in the center.

Prior EP Publication No. 0 147 912 has suggested that thicker strip may be produced by backing the tundish away from the casting wheel to increase the gap therebetween. This has its limits since liquid metal will drain out if the gap becomes too large. Moreover, there are times when a contoured thickness profile may be desired in the strip.

It is an object of the invention to provide a novel metal sheet-casting method.

It is further an object to provide such a method for directly forming metal sheet on a chilled casting drum from a melt in a tundish.

It is also an object to provide such sheet-casting method wherein the thickness of the sheet along its length and width is uniform.

It is also an object to provide a sheet-casting process wherein the thickness profile along its length and across the width is controllable without changing the gap between the tundish and wheel.

In accordance with the objectives of the invention is a method and tundish for casting metal sheet directly from the melt on a chilled casting surface. The apparatus comprises a tundish of the type having a backwall, opposed sidewalls and a floor therebetween and having the sidewalls and floor lip closely adjacent and contoured with the chilled casting surface. The tundish has a portion of the front lip of the floor removed by an offset away from the casting surface allowing a longer contact length of the melt with the casting surface and a thicker sheet in the area of the offset. A portion of the floor lip is retained across the tundish width so that the gap from the wheel remains the same for that remaining portion to retain the liquid metal. A preferred offset for producing a uniform thickness strip across the width is a bow-shaped offset taken from the upper surface of the floor downward. In general, the thickness of the strip is proportional to the depth of the offset. The desired thickness profile may be produced by a particular offset profile.

FIG. 1 is a schematic, elevation view of apparatus for casting sheet directly from the melt.

FIGS. 2(a) and 2(b) are enlarged views of the region between the prior art tundish floor and the casting surface wherein the gap is small and large, respectively.

FIG. 3 shows an enlarged view of the casting region in the invention tundish with contoured lip.

FIG. 4 is a plan view of a tundish having a sine-shaped offset according to the invention which would compensate for the edge-cooling effects on strip thickness.

FIG. 5 is a side, sectional-cutaway view of an alternative tundish offset according to the invention.

FIG. 6 is an isometric view of an alternative tundish having a bow-shaped offset taken from the upper surface of the floor downward.

Known apparatus for forming near-net-shape metal sheet directly from the melt is shown in FIG. 1. The liquid metal 5 is contained in tundish 2. The sidewalls and floor of the tundish 2 on the open end are contoured and closely adjacent the casting surface to deliver a melt layer without leakage between the casting surface and the floor of the tundish.

In FIGS. 2(a) and 2(b), the depth of the melt 5 affects the contact length d.sub.1 along the chilled casting surface 1. As the melt contacts the casting surface for a sufficient time, a solidification front 4 is established between the melt 5 and the sheet 31. A heel 7 may form depending on the separation, x, of the tundish floor 8 and the casting surface, contributing to a slightly longer contact length d.sub.2. The longer the melt contact length, the thicker the sheet. Introducing melt in the tundish to a greater depth will therefore contribute to thicker strip. But too large an inventory also has disadvantages. It is also known that increasing the separation or gap, x, also increases the contact length and strip thickness without increasing the depth of melt in the tundish.

It has been found that molten metal flow through the tundish has an important effect on the dimensional uniformity of the strip produced. Frictional forces along the tundish walls cause melt to move more slowly near the sides of the channel causing more heat loss. Moreover, there is essentially a bi-directional disposal of heat to the casing surface near the cast strip edges (radial and lateral) as opposed to essentially unidirectional (radial) loss near the strip center. This differential cooling results in a solidified strip with a "dog-bone" cross-sectional shape with the edges being thicker than the center. This strip profile is undesirable because it increases the number of processing steps and the cost for producing commercially acceptable rolled strip.

The present invention seeks in general to control the strip thickness to a uniform or desired non-uniform profile. In particular, it seeks to produce a rectangular or slightly convex cross section which is desirable for rolling. This is accomplished by controlling the distance over which solidification of the melt is allowed to occur. This distance is essentially the distance between the point at which the melt first contacts the wheel and the point where the solidified strip emerges from the casting pool. The upper point is therefore related to the upper level of the melt pool. The lower point is related to the level of the tundish floor and lip.

As shown in FIG. 3, the solidification distance or contact length, d, may be controlled by contouring the lip 14 of the tundish floor. The tundish floor has an upper surface 12, a lower surface 13 and a lip 14 contoured to match the casting surface 1. The front face is spaced away from the casting surface by a fixed gap, x. Without the offset or cutout 10 from the lip 14, the depth of the melt 11 is h.sub.x and the solidification distance is d. With the angled offset 10 in the lip, the melt depth increases by an increment .DELTA.h.sub.xi and the solidification distance increases by .DELTA.d.sub.i, at the ith transverse location causing an incremental increase in the strip thickness in that region.

In casting the metal strip, we have found that the thickness, M, is related to the solidification time, t, over the solidification distance, d, by the formula:

The constants C.sub.1 and C.sub.2 are calculated from ##EQU1## where ##EQU2## T.sub.m is the melting and freezing temperature of the metal T.sub.o is the steady state coolant temperature

H.sub.f is the latent heat of fusion

.rho.' is the density of the and melt

C.sub.p ' is the specific heat of the solid strip

k' is the thermal conductivity of the strip

h.sub.c is the effective heat transfer on the casting or solidified layer side of the mold-metal interface and is equal to ##EQU3## H.sub.f is the latent heat of fusion T.sub.s is the wheel-metal interface temperature. h is the global heat transfer coefficient and where

k,.rho. and C.sub.p are the thermal conductivity, density and specific heat of the casting surface.

For a particular aluminum alloy cast on a steel wheel, we have calculated the constants C.sub.1 and C.sub.2 for steady state operation and reduced the equation (1) to

Because the heat loss near the edges of the strip (approximately 2.5 cm) is bi-directional, the heat-transfer coefficients, and therefore the constants C.sub.1 and C.sub.2, will vary. Hypothetically, this may yield the following set of equations defining the conditions across the width of the strip:

The time, t, is related geometrically to the system parameters by: ##EQU4## where .alpha.=the zenith distance to the tundish lip, in degrees

h.sub.x =depth of liquid metal

r=radius of casting wheel

Looking at FIG. 3, a lip offset increases the pool depth by .DELTA.h.sub.xi at the i.sup.th transverse location across the lip and also increases .alpha. by .DELTA..alpha..sub.i so that equation (2) becomes ##EQU5## Since time=distance/velocity and d=r.beta. (r=arc radius=wheel radius; .beta.=the angle transversed, in radians) and since the velocity and wheel radius are fixed, each value of t.sub.i (t+.DELTA.t.sub.i) will define a value of d.sub.i (d+.DELTA.d.sub.i) from which .DELTA..alpha..sub.i can be calculated ##EQU6##

Knowing .DELTA..alpha..sub.i for the i.sup.th location allows the calculation of .DELTA.h.sub.xi using Equation 3. In fact, .DELTA.h.sub.xi is the vertical offset in the tundish lip at the i.sup.th location necessary to produce a thickness M.sub.i in the cast strip. Incremental values of .DELTA.h.sub.xi can be input to NC machinery to automatically contour the desired tundish lip.

The tundish lip may be profiled according to Table 1. To make a 30 cm wide, 0.1 cm thick (nominal) aluminum strip product on a steel wheel with a slight crown (0.097 cm edge symmetric about a 0.102 cm center) using the following process parameters

Casting wheel radius=35.6 cm

Casting speed=41 RPM=91.5 m/min

Cooling water temperature -60.degree. C.

x=45.degree.

FIG. 4 shows an alternative, smooth, sinecontour offset 15 which can produce a desirable strip profile. The following conditions were used to predict the contour shown in Table 2:

Aluminum wheel radius, =35.6 cm

Tundish location, =35.6.degree.

Tundish width, x=35.6 cm

Head height, h=3.5 cm

Wheel speed=100 RPM

Maximum offset desired (center)=0.95 cm to produce nominal 0.685 mm aluminum strip with the edges about 0.05 mm thinner than the center.

FIG. 5 shows an alternative lip design for providing increased solidification distance according to the invention. Such offsets can extend transversely across the entire tundish or could be used only across a portion of the width to effect a variable thickness strip product.

FIG. 6 shows an isometric drawing of another tundish 22 with an alternative lip design in which an arcuate cutout 25 is taken in the lip from the tundish floor upper surface 26 downward. The remaining portions 24 of the lip and 28 of the sidewalls mate with the casting wheel to prevent leakage of the molten metal.