RELATED APPLICATIONS

This application is a counterpart of U.S. Application Serial No. 08/465,948 which is a continuation- in-part of U.S. Patent Application Serial No. 08/272,678.

FIELD OF THE INVENTION

This invention relates to a dual tundish combination for use with twin-roll casters.

BACKGROUND; PRIOR ART

As an alternative to conventional slab casters for the continuous casting of steel slabs, there may be provided for the casting of relatively thin steel strip, a twin-roll caster. The concept of a twin-roll caster is about a century old; variants of such casters have received attention recently for the casting of very thin steel strip not intended to be further reduced from the as-cast thickness. In a twin-roll caster, a pair of horizontally disposed casting rolls of adequately large diameter are rotatably mounted parallel to one another with a slight gap between the two rolls whose width is approximately equal to the thickness of the casting to be made. The rolls rotate in opposite senses downwardly toward the narrowest part of the gap, referred to as the "kissing point". Viewed end on, the left roll rotates clockwise and the right roll counterclockwise. Molten steel is supplied from a tundish above the gap to form a pool just above the gap between the two rolls. The molten steel solidifies as it passes towards and through the gap between the two rolls, and exits as a solid strand having a thickness predetermined by the gap between the two casting rolls.

Such twin-roll casters are illustrated and described, for example, in Japanese published patent specification 62-77151 dated 9 April 1987, Japanese published patent specification 1-249246 dated 4 October 1989, and Japanese published patent specification 3-90261 dated 16 April 1991.

A problem with such twin-roll casters is that unless the depth of the pool of molten steel above the roll gap is carefully controlled, a non-uniform product may result.

A further problem is that as liquid steel is poured from the tundish into the pool of molten steel, some splashing may occur. Droplets of molten steel may strike the casting roll surface before it contacts the pool of molten steel. The droplets may solidify, forming discontinuities in the surface of the cast strand.

A further problem is that casting powder may damage the surface of the cast strand.

A further problem is that lack of control of the atmosphere in the vicinity of the pool of molten steel may lead to unwanted oxidation of the steel.

SUMMARY OF THE INVENTION OF MY COPENDING APPLICATIONS

I have discovered that twin-roll casting may be used to produce a cast strand of a greater thickness range than is normal for twin-roll cast strand, that have extended metallurgical length which will allow the solidification of the strand to be continued at the twin-roll caster exit such that they retain their shape without break-out, and that in combination with suitable downstream reduction using an in¬ line hot rolling process, can produce a finished product of

superior quality relative to the product conventionally produced by twin-roll casters.

To achieve the foregoing objectives, according to my related inventions, I provide a twin-roll caster which in its simplest version is of conventional design except that the gap between the twin casting rolls at the kissing point can be varied over a wider range than is usual in conventional twin-roll casting processes. The gap may be, for example, as wide as about 40 mm or as narrow as about 3 mm, but is preferably selected to be within the range about 5 to about 35 mm.

Immediately downstream of the twin rolls, I provide a stationary mold having a central vertical channel of rectangular cross-section through which the cast strand passes. At the point of entry of the casting into the stationary mold channel, it may lack dimensional stability and may lack sufficient shell strength that it can successfully avoid break-out of molten steel from the casting. However, the shell, in the course of passing through the stationary mold, is cooled sufficiently that the strand becomes wholly or partially solid throughout; dimensional stability is ensured by the" conformation of the shell to the interior cooling faces defining the channel.

This stationary mold in a preferred embodiment is a water-cooled mold having copper cooling faces defining the central open channel structure of the mold. I use the term "copper mold" herein to mean a copper-faced mold. The mold can be of a design essentially similar to that of conventional oscillating copper molds used in slab casters, except that the upper surfaces of my copper mold are concave-shaped to mate with the adjacent generally cylindrical surfaces of the twin casting rolls so that the stationary copper mold may be placed in close proximity to

the twin casting rolls immediately underneath them. The stationary copper mold is provided with interior water channels of the type used in conventional oscillating copper molds.

Immediately downstream of the stationary mold, the cast strand in a preferred embodiment passes through strand containment apparatus comprising a series of support and cooling rolls, preferably segmented rolls, with secondary spray cooling. The cast steel strand is then driven by a pair of drive rolls through a first reduction roll station at which the thickness of the strand is reduced. The drive rolls synchronize the speed of travel of the cast steel strand between the twin-roll caster and the reduction roll station.

The support and cooling rolls (strand containment) downstream of the stationary water-cooled copper mold may also be designed to impart soft reduction to the solidifying cast strand for centerline segregation control. Depending upon space limitations, the first reduction roll station could roll the cast strand while the strand is still travelling vertically, although this is not preferred.

Either before or ^' after passing through this first reduction roll station, the orientation of the cast strand is changed from the vertical to the horizontal using appropriate redirection rolls {bending and unbending rolls to change the orientation of the cast strand) . Following the first reduction, the strand is then passed through at least one and possibly two or more further in-line hot reduction roll stands, preferably finishing stands, before any final downstream operations (such as controlled cooling, shearing and down-coiling) occur.

By arranging the equipment and the processing procedure in the foregoing fashion, I am able to obtain the

thin casting benefit of twin-roll casting, whilst also obtaining the benefit of dimensional uniformity and surface finish of the casting that is typical of the product of larger oscillating mold casters. Further, because the casting is reduced in thickness at least twice and preferably three or more times before being down-coiled for shipment, the metallurgical properties of the steel strand thus produced are quite superior to those obtainable from a conventional twin-roll casting facility.

My related inventions are particularly suitable for the manufacture of steel strand made of any of the following: carbon steel, stainless steel, high-strength low alloy (HS A) steel, and drawing-quality steel.

My related inventions are described and claimed in the following copending U.S. patent applications:

Number Date

SUMMARY OF THE INVENTION OF THIS APPLICATION

Both a primary and secondary tundish are provided for the supply of molten steel to the pool above the twin casting roll gap, the primary tundish being located above the secondary tundish and supplying molten steel to the secondary tundish. The secondary tundish supplies steel to the pool above the twin casting roll gap.

The primary tundish is preferably equipped with flow control devices, which preferably consist of a turbulence inhibitor at the charging area, a baffle and dam, an injector for providing argon injection, and a vortex

killer at the discharge area to help promote inclusion flotation and separation for improving steel cleanliness. A stopper rod or slide gate or the like regulates the liquid steel flow from primary tundish to secondary tundish.

A submerged entry nozzle is preferably provided to shroud the steel flow from the primary tundish to the secondary tundish thereby to minimize reoxidation of the molten steel. An inert atmosphere (argon shield) is preferably employed to displace ambient air at the joint between the primary tundish collector nozzle and the submerged entry nozzle. The submerged entry nozzle is preferably made of high-temperature-resistant refractory material.

The secondary tundish is preferably equipped with a tundish plasma heater of conventional design to improve temperature control during sequence casting. Argon is preferably injected via a suitable gas-permeable well nozzle of the secondary tundish to generate gentle stirring and enhanced float-out of inclusions. The combined effects of plasma heating and inert gas stirring promote inclusion removal, enhance steel cleanliness, and render the molten steel temperature more homogeneous.

An inert atmosphere (argon shield) is preferably provided to displace ambient air at the joint between the primary tundish collector nozzle and the stream guiding shroud. A reducing atmosphere is maintained in order to avoid re-oxidation by atmospheric oxygen.

As another aspect of the invention, I provide an inert or preferably a reducing gaseous atmosphere above the surface of the molten steel pool formed between the casting roll surfaces above the kissing point. This controlled gaseous atmosphere functions as a shield against the entry

of oxygen and eliminates the need for casting powder or the like at the point at which molten steel flows between the casting rolls; this improves both the metallurgical quality of the steel (by eliminating the influx of oxygen) and the surface quality of the cast strand (by eliminating the casting powder, which disadvantageously can be entrained within the liquid steel) .

Mixed gases comprised, for example, of 94% to 98% argon, 1% to 3% CH ₄ , and 1% to 3% C0 ₂ with total pressure slightly above one atmosphere are constantly injected into the space between the anti-splash cover and the meniscus of the liquid steel pool above the twin rolls during the casting operation. The CH. and C0 ₂ are preferably mixed in the molar ratio 1:1 and are assumed to react at 730°C. The complete reaction equilibrium within the system at 730°C yields partial pressure of oxygen in the gases of 2.7 x 10 ^"23 atm. Such reducing gas mixture helps to provide effective protection against the reoxidation of liquid steel.

The foregoing reducing gas mixture can also provide a gas shield (in conjunction with conventional tundish powder, if desired) for the surfaces of the liquid steel in the primary or secondary tundish, to reduce oxidation.

The foregoing controlled gas shield can be used also with other types of caster to advantage,* its use is not necessarily limited to the preferred twin-roll caster herein described.

In contrast to conventional slab casting operations, the molten steel pool is neither covered by casting mold powder, nor contacted by the pouring device via which the pool is continuously supplied. Therefore, interactions with the atmosphere, slag, and refractories can

be significantly reduced, leading to improved cleanliness of the steel.

In combination with the inert gas shield may be advantageously employed an open-pouring guiding shroud extending downwardly from the nozzle well of the secondary tundish and a transversely extending anti-splash cover or splash guard that extends generally horizontally outwardly from the guiding shroud at the underside of the secondary tundish. The guiding shroud, of elongated rectangular cross-section in a preferred embodiment, helps to distribute the liquid steel in a smooth stream uniformly across the length of the casting rolls. The guiding shroud may be a single-nozzle or multiple-nozzle arrangement.

The splash guard extends almost to the surfaces of the casting rolls just above the pool of molten steel confined between the twin casting rolls. The splash guard prevents or at least inhibits droplets of molten steel from splashing onto the casting rolls (which splashing, if it occurred, would tend to impair the surface quality of the cast strand because of surface entrainment of metal droplets) . Further, the splash guard acts as a confinement shroud for the inert gas shield. The splash guard (or anti- splash cover) also minimizes radiation heat loss, since the liquid pool is not covered by casting powder.

The foregoing open-pouring guiding shroud above the molten steel pool formed above the gap in the twin-roll caster, in combination with the protective reducing gas barrier thus shields the steel flow. The shroud may be made of high-temperature resistant refractory material. The use of the steel flow guiding shroud facilitates the obtention of evenly distributed liquid steel across the pool formed above the twin-roll casting rolls, resulting in minimum penetration of the stream into the pool, which can

effectively improve inclusions flotation, thus tending to reduce the entrainment of non-metallic inclusions within the solidifying shell.

The above-described guiding shroud and splash guard are the subject of my copending U.S. patent application Serial No. , filed on

The use of a two-stage tundish arrangement itself contributes to lower stream momentum as the stream of liquid steel enters the pool above the gap between the twin casting rolls. This facilitates even flow and minimizes turbulence. These desirable effects are enhanced by the use of the rectangular single-nozzle or multiple-nozzle guiding shroud. The flow rate of liquid steel is regulated by the ferrostatic height and the cross-sectional area of the guiding shroud. Note also that in the event of an emergency, flow from the primary tundish to the secondary tundish can be shut off, leaving at most the contents of the secondary tundish to cause any damage. These constituent elements of the preferred design in conjunction with the open pool above the twin casting roll gap and the use of a controlled gaseous atmosphere thereabove promote the even casting of clean steel relatively free of unwanted inclusions at a relatively even casting temperature.

SUMMARY OF THE DRAWINGS

Figure 1 is a schematic end elevation view, partly in section, of a preferred embodiment of the primary and secondary tundishes, twin rolls and stationary mold for use with a caster arranged in accordance with the principles of the present invention.

Figure 2 is a schematic detailed isometric view of the elements of Figure 1 shown in preferred conjunction in accordance with the principles of the invention.

Figure 3 is a schematic isometric view of a single-port version of the secondary tundish of Figure 2.

Figure 4 is a top view of the secondary tundish of Figure 3.

Figure 5 is a schematic isometric view of a multiple-port version of the secondary tundish of Figure 2.

Figure 6 is a top view of the secondary tundish of Figure 5.

DETAILED DESCRIPTION WITH REFERENCE TO THE DRAWINGS

Molten steel is supplied from a primary tundish 1 to a secondary tundish 3 and thence via a guiding shroud 4 to form a pool of molten steel 53 just above the gap

(kissing point) 55 formed between a pair of parallel horizontally aligned casting rolls 57, 59 rotating in opposite senses, the roll 57 rotating clockwise, and the roll 59 counterclockwise, as seen in the drawings.

Framework, bearings, mountings, valves, etc. are omitted from the drawings for the purposes of clarity and simplicity.

The casting rolls 57 and 59 of the twin-roll caster, referred to generally by reference numeral 7, have copper peripheral cylindrical surfaces. Twin-roll casters are well-known in the industry; a useful review can be found in the paper by Kasama et al . , "Twin Drum Casting Process for Stainless Steel Strand", Proceedings of SNRC-90 Conference, 14-19 October 1990, Pohang, Korea, held by The

Korean Institute of Metals and The Institute of Metals, UK, at pp. 643-652. See also Cramb, "New Steel Casting Process for Thin Slab and Strand: A Historical Perspective", Iron and Steelmaker Vol. 20 No. 7, 1988, pp. 45-68. Such twin- roll casters preferably have slightly concave crown profiles in conformity with preferred practice so as to give the cast strand a slight convex profile (positive strand crown profile) . The convex profile is desirable for uniform deformation of the hot strand during subsequent hot rolling reduction (see, e.g. Chiang, "Development and Application of

Pass Design Models at IPSCO's Steckel Hot Strand Mill"

(1992) , 33rd MWSP Conference Proceedings, ISS-AIME, Vol. 29.

The rolls 57, 59 may be kept within profile specifications by on-line peripheral roll grinders 8 of conventional design.

The twin-roll caster comprising casting rolls 57 and 59, is used to cast strand ranging from about 5 mm to about 35 mm in thickness, or, less economically, sized outside these preferred dimensions to a lower limit of about 3 mm and an upper limit of about 40-50 mm. The casting may preferably be about 900 to about 1800 mm in width, or somewhat outside these dimensions. This as-cast strand may preferably be subsequently processed by in-line hot rolling stands (as described in my copending U.S. patent application Serial No. , filed on , to achieve finished strand thickness ranging from about 1.5 mm to about 12 mm, assuming the conventional 3-to-l reduction of the initial casting. The speed of rotation of the casting rolls 57, 59 is preferably selected to range from about 1.5 rpm to about 12 rpm, the latter for castings of about 5 mm thickness and the former for castings of about 35 mm thickness. Cooling water flow through the rolls 57, 59 is set at about 500 GPM to 1000 GPM per roll to provide optimum cooling effect for good strand surface quality, and is adjusted according to the thickness of the casting.

Within the primary tundish 1 is a continuing supply of molten steel 61 (of, say, 30 tons within tundish 1) replenished on a steady basis from a ladle of molten steel (not shown) . Although not shown in the drawings, the primary tundish 1 is preferably equipped with suitable flow control devices, which may, for example, consist of a turbulence inhibitor at the charging area, a baffle and dam, an injector for providing argon injection, and a vortex killer at the discharge area to help promote flotation and separation of inclusions for improving steel cleanliness . These devices are all described in my paper "Water Modelling of IPSCO's Slab Caster Tundish" published at page 437 ff . of the 1992 Steelmaking Conference Proceedings.

A stopper rod or slide gate of conventional design

(not shown) , e.g. the 13QC model sold by Stopinc AG, regulates the liquid steel flow from the primary tundish 1 to a secondary tundish 3. The molten steel flows from the primary tundish 1 to the secondary tundish 3 via a well exit port 48 and associated submerged entry nozzle 2. At the joint between the primary tundish exit port and the submerged entry nozzle 2, an inert atmosphere (argon shield) is preferably provided by means of an argon injection device of conventional design. Such injection devices are used to displace any ambient air (and particularly oxygen) at the joint. The submerged entry nozzle 2 of conventional design, preferably made of high-temperature-resistant refractory material such as high-alumina graphite, shrouds the steel flow from the primary tundish 1 to the secondary tundish 3 to reduce re-oxidation of the molten steel.

A pool 63 of molten steel within the secondary tundish 3 is continuously replenished from the primary tundish 1. The secondary tundish 3 may have, say, a capacity of five tons. The secondary tundish 3 is preferably equipped with a tundish plasma heater (not shown)

of conventional design (e.g. of the type supplied by Plasma Energy Corp. and installed in the Nucor Steel plant in Norfolk, Nebraska) to improve temperature control within the range of about 5°C of target superheat temperature during sequence casting operations. For gentle stirring of the steel in the secondary tundish 3, either conventional argon stirring by means of argon injected into the well exit port 51 of the secondary tundish 3 is provided, or else induction stirring devices of the conventional type such as the EMS stirrers supplied by ABB Metallurgy may be used to generate gentle stirring of the molten steel, and to enhance the floating out of the steel of unwanted inclusions.

A guiding shroud 4 of rectangular cross-section fixed to the underside of the secondary tundish 3 and communicating with the exit port 51 of the secondary tundish

3 guides the flow of steel into the pool of molten steel 53 formed immediately above the gap 55 between twin casting rolls 57 and 59. The transverse area dimension of the guiding shroud 4 at the exit port 51 from the secondary tundish 3 is preferably about 5 mm by about 600 mm, which enables the pouring of approximately four tons per minute of liquid steel from the secondary tundish 3 into the pool 53.

The guiding shroud 4 tends to isolate the incoming steel from ambient oxygen. Inert gas or a reducing gas or a combination of both is injected above the pool 53 via injector nozzles 81 to prevent oxygen from gaining access to the surface of the molten steel pool 53.

Alternatively, a series of horizontally aligned spaced guiding shrouds 43 could be provided, each communicating with a discrete exit port 41 on the underside of tundish 3 (Figures 5, 6) . The provision of a multiple guiding shroud arrangement instead of the single guiding shroud arrangement of Figures 3, 4 may facilitate uniform pouring from the tundish 3 into the pool 53 above kissing

point 55 of the twin rolls 57, 59. The cross-sectional area of the guiding shrouds 43 will be selected to give the desired pour rate; in the three-port example of Figures 5, 6, dimensions of about 5 mm by about 200 mm may be suitable for about a 4-ton-per-minute casting rate.

The presence of the guiding shroud 4 (or guiding shrouds 43) enables more precise flow control of the stream of liquid steel into the pool 53 than would otherwise be possible. The greater cross-sectional area of such shroud 4 as compared with that of conventional tundish drain outlets of circular configuration permits equivalent flow rate with reduced penetration of the stream of steel into the pool 53, leading to less turbulence than with conventional outlets. The large cross-sectional area of guiding shroud 4 combined with a low ferrostatic height for the secondary tundish 3 significantly reduces the linear velocity of liquid steel poured into the pool 53. Because the pouring of the steel into pool 53 is open (as distinct from submerged) , the flow momentum tends to be dissipated because of the surface turbulence dumping effect . The use of the dual tundish arrangement, of course, itself contributes to lower momentum of the steel flow into the pool 53 as' compared with a single-tundish arrangement. Note also that the use of two tundishes facilitates the flotation of unwanted inclusions, since the inclusions have two opportunities to float toward the surface of the molten steel .

Attached to the underside of the secondary tundish

3 and extending generally horizontally outwardly from and spaced by a short distance from the guiding shroud 4 are a symmetrical pair of shield plates 75, 77 forming a splash guard 5. The plates 75, 77 are lined at least on the underside with a suitable high-temperature-resistant refractory material. The plates 75, 77 are preferably

spaced about 5 cm above the meniscus of the steel pool 53. Penetrating through the shield plates 75, 77 are an array of gas injector nozzles 81, only two of which are apparent in Figures 3 and 5, but as many as perhaps a dozen or so are 5 spaced along the length of the shield plates 75, 77. The bent edges 71, 79 are formed and dimensioned to lie close to the peripheries of the twin rolls 57, 59 just above the surface of the liquid steel pool 53 (see Figure 1) .

10 The anti-splash cover or splash guard 5 is designed to prevent splashed metal droplets from sticking to either of the rotating twin rolls 57, 59. Such spray of droplets is often caused by the impact of the liquid steel stream on the surface of the liquid pool underneath. Another purpose of

15 the splash guard 5 is to minimize radiation heat loss, since, in contrast to conventional designs, the liquid pool 53 is not covered by casting powder. Nor is the pool 53 in contact with a pouring nozzle or shroud. Therefore, interactions with the atmosphere, slag, and refractories can

20 be significantly reduced, leading to improved cleanliness of

^* the steel.

Mixed gases comprised of about 94% to 98% argon, 1% to 3% CH ₄ , and 1% to 3% C0 ₂ supplied at a total pressure

25 slightly above one atmosphere are constantly injected into the space between the anti-splash cover 5 and the liquid steel pool 53 during the casting operation. These gases enter the space above the pool 53 via the injector nozzles 81. They are prevented from rapidly leaving this space by

30 the close spacing of the edges 71, 79 of the splash guard shield plates 75, 77 to the peripheries of rolls 57, 59.

The CH ₄ and C0 ₂ thus supplied are mixed in the molar ratio 1:1 and are assumed to react at 730°C to form CO

35. and H ₂ , both reducing gases. The complete reaction equilibrium within the system at 730°C yields a calculated

partial pressure of oxygen in the gases of 2.7 X 10 ^"23 atm. Such a reducing gas mixture can provide effective protection against the reoxidation of liquid steel in the pool 53.

Knowing the liquid steel head (ferrostatic height) in the secondary tundish 3 and the dimensions of the kissing-point gap 55 and the well nozzle 51, then from the mass balance viewpoint under steady-state conditions at a given casting speed, it is possible to design the described apparatus to provide a predetermined quantity of steel 63 in the secondary tundish 3 that generates a pool of liquid steel 53 over the desired surface contact area of the twin casting rolls 57, 59. The surface contact area parameter is usually expressed as the mold-level angle A subtended by the meniscus of the pool 53 and the kissing-point gap 55. This angle A should preferably be selected to lie in the range about 30° to about 45°. The flow rate of liquid steel required is governed by ferrostatic height and cross- sectional area of the guiding shroud nozzle (s) .

The described dual tundish arrangement may be used with various state-of-the-art features of tundish design generally, e.g. electromagnetic valves, plasma heaters, etc.

Located between the ends of the twin casting rolls

57, 59 are side dams 83 whose concave arcuate sides 85, 87 conform in shape and dimension to the cylindrical peripheries of the rolls 57, 59. The side dams 83 serve to confine the ends of the steel pool 53. The dams 83 are preferably made of high-temperature-resistant refractory material. The top edge 89 of each of the dams 83 must be above the level of the meniscus of the steel pool 53 sufficiently to prevent any overflow, and should extend as close as feasible to the splash guard 5 so as to minimize the loss of the inert gas atmosphere. The bottom edge 91 should extend below the kissing-point gap 55 to just above

the top edges of the stationary mold 10, so as to minimize the risk of any break-out between the dam 83 and the stationary mold 10. The dams 83 are designed to be movable transversely in either direction. The dam 83 illustrated in Figure 2 is at the outer limit of its possible transverse movement. The dams 8, 3 may move inwardly from their extreme positions at the ends of the rolls 57, 59 to reduce the width of the cast strand. Means (not shown) , such as a suitable conventional hydraulic piston/cylinder arrangement, may be provided to adjust the spacing between the dams 83 to accommodate varying widths of strand. Given the absence of casting powder, the provisions of such lubricant is especially desirable, and may in some circumstances be necessary.

Rape-seed oil or other suitable lubricant is applied to the surface of each of the twin casting rolls 57, 59 via lubricant injectors 6. The lubricant tends to minimize the risk of adherence of steel droplets to the surfaces of the casting rolls 57 and 59, and tends to prevent sticker-type breakouts of the cast strand.

As the molten steel passes from the top of pool 53 to the gap 55, it begins to solidify. If the gap 55 is very narrow, say less than about 5 mm, the steel may be completely solidified at or near the kissing point between rolls 57, 59. However, at wider gap dimensions, the still hot, liquid core of the steel as it emerges downstream of the gap 55 will not permit the strand reliably to retain its shape; the risk of break-out would be high. This fact has limited the use of conventional twin-roll casters to cast strand thicknesses of less than about 5 mm.

Positioned immediately downstream and underneath of the rolls 57 and 59 is a stationary mold 10 having a central channel 52 of rectangular cross-section whose narrow

dimension is approximately equal to or very slightly smaller than the dimension of the gap 55 between the twin casting rolls 57 and 59. The width of the channel 52 may taper very slightly inwardly from top to bottom to accommodate thermal contraction and solidification shrinkage of the steel strand as it solidifies; the gap width may receive fine adjustment by machining the surfaces of the stationary copper mold 10.

The stationary mold 10 is preferably a water- cooled copper mold, i.e. its faces forming the interior channel 65 are formed of copper; the balance of the mold structure may be made of steel. The mold 10 is shaped so that its upper concave surfaces 69 lie as close as possible to the casting rolls 57 and 59 above, and in particular so that the entry mouth 52 of the mold channel 65 is as close as possible to the kissing-point 55 between the casting rolls 57 and 59. The flow of mold cooling water may be adjusted so that heat flux extraction in the range of about

5 to about 30 cal/cm ² /sec is obtained. This range should be satisfactory for the range of casting thicknesses for which the equipment is designed.

For the casting of thin strands, the stationary mold 10 may not be necessary. If the strand is solid as it leaves the twin casting rolls 57, 59, there is no need for the stationary mold 10, which can be removed and/or by¬ passed.

Cooling of the molten steel occurs over that portion of the peripheral cylindrical surface of each of the rolls 57, 59 subtended by angle A (Figure 3) , and by the interior vertical faces 73 of the stationary mold 10. Further cooling may occur in a strand containment and secondary spray cooling station 11, as described in detail in my copending U.S. patent application Serial No. , filed on

It can be seen from Figure 1 that the vertical faces 73 of the stationary mold provide a cooling area that is about equal to the cooling area provided by the cylindrical surface subtended by angle A of each of the twin casting rolls 57, 59. However, the ratio of cooling surface area of stationary mold to twin-roll caster cooling surface area, and the ratio of both to the strand containment cooling area to be described further below, may vary considerably according to the designer's preference. In any case, the additional provision of the stationary mold to the layout can substantially increase the available primary cooling area for the molten steel being cast, as compared with conventional twin-roll caster design. This enables much wider gaps 55, 65 to be present between the twin rolls 57, 59 and the two opposed cooling blocks 64, 66 of the stationary caster 10 than is possible using conventional design.

For thinner castings, the end walls 45 of the stationary mold 10 may be omitted if desired, and a cooling water spray device (not shown) substituted.

While reference herein is made to the mold 10 as being a "stationary" mold, it is to be understood that the two opposed cooling blocks 64, 66 of the stationary mold 10 can, in fact, be moved towards and away from one another to accommodate varying thicknesses of casting. The same, of course, is true for the twin rolls 57 and 59; the gap 55 may be adjusted according to the casting thickness desired. Although the apparatus according to the invention can be used for making castings with a thickness as thin as about 5 mm, some of the principal advantages of the invention are most markedly obtained when the thickness of the casting is relatively large, in about the 35 mm range.

Substitutions, modifications, omissions and refinements may occur to those skilled in the art. The scope of my invention is as defined in the appended claims.