GB1596395A - Method of continuous casting of steels or metal alloys with segregation tendancy and apparatus for carrying out the method - Google Patents

Description

(54) METHOD OF CONTINUOUS CASTING OF STEELS ORN METAL ALLOYS WITH SEGREGATION TENDENCY AND APPARATUS FOR CARRYING OUT THE METHOD (71) We,JERNKoNoRET FoRsKNING- SAVDELNINGEN, a Swedish organisation, Box 1721,111 87 Stockholm, Sweden, do hereby declare the invention, for which we pray, that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a method of continuous chill casting of steel or other metallic material in the form of a downwardly cast strand with the purpose of preventing or minimising the formation of segregations.

With continuous casting of highearbon steels, for example ball bearing steels, highspeed tool steels and also other steels with high carbon content, distinctive carbide segregations appear which render the material unsuitable for many fields of application. The same kind of carbide segregations also can arise when the aforesaid steels are cast in conventional chills and with ESR-remelting at high melting at high melting rates.

Carbide segregations are formed, for instance, during the solidification of the inner parts of a steel ingot. Due to the large solidification intervals of the steels, relatively thick zones of semisolidified material are formed. In said zones dendrites form a porous network of solidified material with a lower than average carbon content and a lower than average content of impurities in the material. In the intermediate spaces between the dendrites, thus, residual melts with higher carbon content are located.

During the solidification the material shrinks, partly as solidification shrinkage of about 4% and partly as cooling shrinkage in the material already solidified.

The material solidifies from the outer surfaces inward to the centre of the material. This results in several solidification fronts existing during the solidification and growing toward the material centre. With continuous casting, furthermore, a strand with unsolidified material in the centre moves from a chill downward.

Depending on the dimension of the strand and its casting rate, the solidification zone, i.e. the zone with semi-solidified material, varies in the longitudinal direction of the strand with respect to length and other dimensions. When the solidification zone has an unfavourable configuration, i.e. when it is long and thick, high stresses arise between solidification fronts having met. These stresses arise, because the outer surfaces of the strand are solidified and only shrink because of cooling shrinkage, while the interior of the strand shrinks because of the greater solidification shrinkage. As a result of these stresses, the fronts separate. The shrinkage gives rise to a vacuum, which sucks down the melt through the porous semi-solidified material. This melt is enriched with impurities and alloying elements and, consequently, large carbide segregations are formed in the centre of the strand. Corresponding conditions prevail with all alloys with great solidification intervals and give rise to segregations.

When the solidification zone is long, and the material solidifies and shrinks in central portions of the strand, relatively large amounts of melt must be transported to the semi-solidified zone.

As a result thereof, substantial macrosegregations arise which form pores and cracks in the central portion. With continuous casting it is known that carbide segregations can be reduced by carrying out the casting very slowly. The casting rate, however, in that case must be reduced so much that the process is uneconomic.

According to one aspect of this invention there is provided a method of continuous chill casting of steel or other metallic material in the form of a downwardly cast strand wherein during solidification thereof a predetermined elongate region of the cast strand, which comprises solid, semi-solidified and liquid regions and which extends from a casting chill used for the casting to the point where the strand is fully solidified, is caused to become directly or indirectly progressively deformed plastically by the application of a plurality of external deforming means extending along the said region in such a manner that the reduction in cross-section of solidified metallic material caused by cooling shrinkage as it continues to cool is supplemented by the reduction in crosssection of the said solidified material caused by the external deforming means so that the combined reduction is substantially equivalent to or slightly greater than the solidification shrinkage of adjacent molten metallic material as it changes into the solid state, and any upward or downward transport of melt in the solidifying strand is substantially avoided, thereby preventing or minimising the formation of segregations. The cast strand may have an elongate barrel-shaped cross-section with a pair of opposite longer convex sides, which convex sides are deformed plastically by the action of plane rolls thereon. The casting chill may be so shaped that the cast strand has an elongate rectangular cross-section, and opposite longer sides of the strand are deformed plastically by the action of rolls thereon, the rolls having a diameter decreasing from the centre to each end. The casting chill may be so shaped that the cast strand has a square, octagonal or other polygonal or circular cross-section, and opposite edge regions or portions of the strand are deformed plastically by the action of rolls thereon, the rolls having a centrally recessed or concave portion to accommodate the said opposite edges or portions of the strand. The external deforming means may constitute forced cooling means which effectively deform inner semi-solidified parts of the strand plastically by cooling shrinkage of outer parts of the strand. The casting chill may be so shaped that the cast strand has an elongate rectangular cross-section, and opposite longer sides of the strand are deformed plastically by the said forced cooling means. The said forced cooling means may be applied to central portions of the said opposite longer sides of the strand.

According to another aspect of the invention there is provided an apparatus for carrying out the method defined in the above paragraph, the apparatus comprising a casting chill and means for chill casting steel or other metallic material in the form of a downwardly cast strand in such a manner that during solidification thereof a predetermined elongate region of the cast strand is adapted to comprise solid, semisolidified and liquid regions and to extend from the casting chill to the point where the strand will become fully solidified, and a plurality of external deforming means which are arranged to extend along the said region so as to cause the said region to become directly or indirectly progressively deformed plastically by the application of a plurality of external deforming means extending along the said region in such a manner that the reduction in cross-section of solidified metallic material caused by cooling shrinkage as it continues to cool is supplemented by the reduction in cross-section of the said solidified material caused by the external deforming means so that the combined reduction is substantially equivalent to or slightly greater than the solidification shrinkage of adjacent molten metallic material as it changes into the solid state, and any upward or downward transport of melt in the solidifying strand is substantially avoided, thereby preventing or minimising the formation of segregations. The external deforming means may comrpise roll pairs located one after the other in the direction of a strand movement, the distance between the two rolls in each pair decreasing with increase in distance from the casting chill. The casting chill may be so shaped that the cast strand has an elongate rectangular cross-section, and the external deforming means constitute forced cooling means comprising a plurality of spray nozzles arranged to spray coolant against the strand so that in operation the forced cooling means adapted to be applied to the opposite longer sides of the strand will effectively deform inner semi-solidified parts of the strand plastically be cooling shrinkage of outer parts of the strand. The casting chill may be so shaped that the cast strand has a square, octagonal or other polygonal or circular crosssection, and the external deforming means constitute forced cooling means comprising a plurality of spray nozzles arranged to spray coolant against the strand so that in operation the forced cooling means adapted to be applied around the strand will effectively deform inner semi-solidified parts of the strand plastically by cooling shrinkage of outer parts of the strand.

By way of example preferred forms of the invention will now be described in greater detail with reference to the accompanying drawings, in which: Figures 1 and 2 each are a longitudinal section through a strand and associated chill; Figure 3 is a longitudinal section through a strand and associated chill and a device for effecting plastic deformation of the strand; Figures 4-6 are each a cross-section of a strand during its plastic working; Figure 7 is a longitudinal section of such a strand.

Figures 8-9 each are a cross-section of a strand in different solidification phases, and Figure 10 is a longitudinal section of a strand The formation of suctions, stresses and cracks in a semi-solidified area depends on the configuration of the solidification zone. Figure 1 shows a solidification zone having a favourable configuration with respect to suctions, stresses and cracks, because the semi-solidified material 2 has a short extension in the vertical direction in Figure 1, i.e. in the longitudinal direction of the strand. The semi-solidified material 2 is surrounded by molten material 1 and solidified material 3. A chill 4 encloses the strand 1, 2, 3. A solidification zone of the configuration shown in Figure 1 arises with a very low-rate continuous casting, with normal ESRrecasting and with the casting of a thick, short ingot. Figure 2 shows a solidification zone having an unfavourable configuration, because the semi-solidified material 2 has a large vertical extension. This type of solidification zone is formed with normal and rapid continuous casting, at high-rate ESR-remelting and with the casting of a long, narrow ingot. When a material, which is cast by normal or rapid continuous casting, see Figure 2, shrinks at the centre, relatively large amounts of melt 1 are transported downward from above, in Figure 2, due to the relatively large area with semisolidified material. As a result thereof, substantial segregations arise, as mentioned above, over a larger area, in the form of socalled macrosegregations, which give rise to pores and cracks in the central portion. The process described with reference to Figure 2 is the normal process with continuous casting. The casting rate is so high, that the solidification zone is relatively long. The above known technique for preventing carbide segregations consists of low-rate casting whereby a small solidification zone, according to Figure 2, is formed. This process, however, is unfavourable from an economic aspect.

With the present invention, the strand is deformed plastically so that the area reduction substantially corresponds to or slightly exceeds the solidification shrinkage inthe material. The plastic deformation of the strand is effected substantially in the place where the strand consists of both semi-solidified and solidified material. When the central portions solidify and this material shrinks by solidification, the strand is subjected to a reduced working so that its cross-section area is reduced to a dimension corresponding to the area of a solidified and entirely welded-together material over the cross-section of the strand. Due to this process, melt cannot be sucked down into the semisolidified material 2. Consequently, the formation of macro-segregations as well as of pores and cracks in the central portions is prevented.

In Figure 3 a device is shown, by which a working operation for deforming the strand can be carried out. The molten metal is poured down through the chill 4 and solidifies substantially immediately on the surface. The solidifid strand is passed down and out of the chill 4, and thereafter is introduced between a plurality of roll pairs 5. Each of said roll pairs 5 has a spaced relationship between the rolls which brings about an area reduction corresponding to the solidification shrinkage occurring in the strand at each roll pair. The strand, thus, from the first roll pair and downward is entirely welded-together at its centre. After the last roll pair, the strand is entirely solidified. Due to this successive working, the molten material 1 (socalled "melt") will not be sucked down into the semi-solidified material 2 when the solidification shrinkage commences.

With the continuous casting of workpieces with rectangular cross-section, socalled slabs, the corners and portions adjacent thereto are cooled much more rapidly than the remaining part of the strand. As a result thereof, the solidification shrinkage, which causes the sucking down of melt 1 into the semi-solidified material 2, takes place in the central strand portions, which solidify later. This implies that only the broad sides of a strand with rectangular cross-section should be worked. It is thereby accentuated that a strand, due to the stronger cooling at the corners, tends to assume a greater thickness at the centre of the broad sides where the material is hotter.

Figure 4 shows in a schematic way a device according to an embodiment of the invention whereby only a portion of the broad sides of a strand is intended to be worked. A strand 6 with generally convex broad sides is cast in a chill 4 (see Figure 3) and worked between two plane rolls 8, 9. Thereby only that portion of the convex broad sides is worked which has contact with the plane rolls. After the working, the strand has a reduced cross-section area, because the strand has assumed a less convex configuration while the areas at the corners of the strand are substantially unworked. The convexity of the strand can be adjusted at casting so that, as a result of the necessary reduction of the cross-section by working with rolls, the strand after the working has a rectangular cross-section.

The reduction of the strand according to the embodiments described above and in the following must be so great, that it slightly exceeds the reduction in area which corresponds to the solidification shrinkage going on.

The reduction must be carried out in several steps, as indicated in Figure 3, so that a substantially continuous area reduction is obtained which is adjusted to and corresponds to the solidification shrinkage. Tensile stresses in the solidifying material are hereby avoided and only moderate compressive stresses are obtained. The number or reduction steps is determined by practical factors, especially by the casting rate and, thereby, the length of the solidification zone. In high-speed continuous casting machines, with a solidification zone length of up to 20 meters, the working can take place in 20 to 40 steps, while in slower operating machines, for example and ESR-machine, the working must be carried out in a few steps.

A suitable total reduction of the crosssectional area of the strand generally is 1-10%, preferably 2-6%. For steel, a suitable reduction generally is 4%.

The rolls 8,9 are arranged to rotate at the same circumferential speed as the rate of the cast strand at said roll pair. A plurality of roll pairs similar to the roll pair 8, 9 can be positioned with different spaced relationship to the chill, as shown in Figure 3.

Another embodiment is shown in Figure 5, according to which the strand 6 is cast with rectangular cross-section and plane broad sides, and the working is carried out with rolls 10, 11, which are cambered, i.e. so designed as to have a diameter decreasing from the centre to both ends.

According to this embodiment, a strand is obtained after the working which has the smallest thickness at its centre and increasing thickness to the short sides of the substantially rectangular cross-section of the strand. In general, the above information with respect to the plane rolls 8, 9 according to Figure 4 and the roll pairs in Figure 5 applies also to this embodiment. A corresponding working of strands with square cross-section, octagonal cross-section, round cross-section or a crosssection of another shape can be carried out by means of tools, which enclose the strand as completely as possible, because the cooling of the strand with such cross-sections is more symmetrical than at strands with rectangular cross-section.

In order to illustrate this, Figure 6 shows schematically a device for working a strand with substantially square cross-section. The strand 6 is worked by means of two rolls 12, 13, which are provided with grooves, the configuration of which corresponds to the shape of the strand at two diagonally opposite corners. The grooves are given such a depth, that they together substantially enclose the strand, which is being worked, also along its sides. When several roll pairs similar to the rolls 12, 13 are arranged one after the other, the axles of such roll pairs can form an angle of 90" with each other in order to work the strand symmetrically. A further embodiment of the invention is shown schematically in Figure 7. A strand 6 is worked here by means of two opposed reciprocating forging tools 15, 16 with working surfaces facing toward each other, which surfaces between themselves form a space adjusted to the shape of the strand and to the type of working, to which the strand is to be subjected. Said space tapers to wedge shape in the direction of strand movement in order to subject the strand to the desired reduction with respect to its cross-section area. The arrows 17, 18 in Figure 7 indicate the direction of movement of the tools 15, 16. With this device, the strand 6 is advanced one step when the forging tools 15, 16 move away from each other, and is deformed when said tools move toward each other. By working the strand by means of forging tools 15, 17 conical in the longitudinal direction of the strand 6, an almost continuous reduction of the crosssection of the strand is obtained.

The working surfaces of the forging tools 15, 16 can, perpendicularly to the longitudinal direction of the strand 6, be formed planar, convex or concave, depending on the crosssectional shape of the strand 6.

According to a further embodiment of the invention, the reduction of the cross-section of the strand 6 is effected by controlled cooling of the strand 6.

Immediately after its leaving the chill 4, (Figure 10), the strand 6 has a cross-section corresponding to the inner form of the chill 4.

In Figure 8 a rectangular ross-section of a strand is shown as an example. The corners 19 and the areas immediately adjacent thereto are colder than the centre on the broad sides 20 of the strand 6 and the material inside thereof.

The solidification process is shown by way of example in Figure 8, with solidified material 3 at the colder portions and semi-solidified material 1 in the interior of the strand. Due to this temperature difference, the strand is thinner adjacent the corners 19 than at its centre, because solidification shrinkage and cooling shrinkage have occurred adjacent the corners 19, whereby the strand assumes a convex appearance as shown in Figure 9.

According to this embodiment, a reduction of the cross-section area of the strand 6 is obtained thereby, that the broad sides of the strand 6 are subjected to forced cooling, whereby the surface layer of the convex portions and solidified material 21 inside thereof are contracted and deform the centrally located semi-solidified material. Thereby the necessary deformation of the strand is obtained. The cooling, thus, is started during the final solidification phase of the strand, as appears from above.

This embodiment can be applied also to strands with other cross-sections. In the case of square, octagonal, round or like shape of the strand, the forced cooling is carried out so that all sides or outer surfaces of the strand are cooled. This ensures, that the entire outer shell of the strand shrinks as a result of the cooling shrinkage, whereby the necessary reduction of the cross-section takes place and the inner semisolidified strand materials is deformed.

The forced cooling is effected by a plurality of nozzles 22 (Figure 10), which eject coolant 23 under the required pressure against the strand 6 in the above indicated places. The coolant may be water, water-air mixture of steam.

By "forced cooling" as described above with reference to Figure 10 it is to be understood that the cooling is so effective that outer parts of the strand 6 by cooling shrinkage deform inner semi-solidified parts of the strand plastically.

WHAT WE CLAIM IS: 1. A method of continuous chill casting of steel or other metallic material in the form of a downwardly cast strand wherein during solidification thereof a predetermined elongate region of the cast strand, which comprises solid, semi-solidified and liquid regions and which extends from a casting chill used for the casting to the point where the strand is fully solidified, is caused to become directly or indirectly progressively deformed plastically by the application of a plurality of external deforming means extending along the said region in such a manner that the reduction in crosssection of solidified metallic material caused by cooling shrinkage as it continues to cool is supplemented by the reduction in cross-section of the said solidified material caused by the external deforming means so that the combined reduction is substantially equivalent to or slightly greater than the solidification shrinkage

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (22)

**WARNING** start of CLMS field may overlap end of DESC **. thickness to the short sides of the substantially rectangular cross-section of the strand. In general, the above information with respect to the plane rolls 8, 9 according to Figure 4 and the roll pairs in Figure 5 applies also to this embodiment. A corresponding working of strands with square cross-section, octagonal cross-section, round cross-section or a crosssection of another shape can be carried out by means of tools, which enclose the strand as completely as possible, because the cooling of the strand with such cross-sections is more symmetrical than at strands with rectangular cross-section. In order to illustrate this, Figure 6 shows schematically a device for working a strand with substantially square cross-section. The strand 6 is worked by means of two rolls 12, 13, which are provided with grooves, the configuration of which corresponds to the shape of the strand at two diagonally opposite corners. The grooves are given such a depth, that they together substantially enclose the strand, which is being worked, also along its sides. When several roll pairs similar to the rolls 12, 13 are arranged one after the other, the axles of such roll pairs can form an angle of 90" with each other in order to work the strand symmetrically. A further embodiment of the invention is shown schematically in Figure 7. A strand 6 is worked here by means of two opposed reciprocating forging tools 15, 16 with working surfaces facing toward each other, which surfaces between themselves form a space adjusted to the shape of the strand and to the type of working, to which the strand is to be subjected. Said space tapers to wedge shape in the direction of strand movement in order to subject the strand to the desired reduction with respect to its cross-section area. The arrows 17, 18 in Figure 7 indicate the direction of movement of the tools 15, 16. With this device, the strand 6 is advanced one step when the forging tools 15, 16 move away from each other, and is deformed when said tools move toward each other. By working the strand by means of forging tools 15, 17 conical in the longitudinal direction of the strand 6, an almost continuous reduction of the crosssection of the strand is obtained. The working surfaces of the forging tools 15, 16 can, perpendicularly to the longitudinal direction of the strand 6, be formed planar, convex or concave, depending on the crosssectional shape of the strand 6. According to a further embodiment of the invention, the reduction of the cross-section of the strand 6 is effected by controlled cooling of the strand 6. Immediately after its leaving the chill 4, (Figure 10), the strand 6 has a cross-section corresponding to the inner form of the chill 4. In Figure 8 a rectangular ross-section of a strand is shown as an example. The corners 19 and the areas immediately adjacent thereto are colder than the centre on the broad sides 20 of the strand 6 and the material inside thereof. The solidification process is shown by way of example in Figure 8, with solidified material 3 at the colder portions and semi-solidified material 1 in the interior of the strand. Due to this temperature difference, the strand is thinner adjacent the corners 19 than at its centre, because solidification shrinkage and cooling shrinkage have occurred adjacent the corners 19, whereby the strand assumes a convex appearance as shown in Figure 9. According to this embodiment, a reduction of the cross-section area of the strand 6 is obtained thereby, that the broad sides of the strand 6 are subjected to forced cooling, whereby the surface layer of the convex portions and solidified material 21 inside thereof are contracted and deform the centrally located semi-solidified material. Thereby the necessary deformation of the strand is obtained. The cooling, thus, is started during the final solidification phase of the strand, as appears from above. This embodiment can be applied also to strands with other cross-sections. In the case of square, octagonal, round or like shape of the strand, the forced cooling is carried out so that all sides or outer surfaces of the strand are cooled. This ensures, that the entire outer shell of the strand shrinks as a result of the cooling shrinkage, whereby the necessary reduction of the cross-section takes place and the inner semisolidified strand materials is deformed. The forced cooling is effected by a plurality of nozzles 22 (Figure 10), which eject coolant 23 under the required pressure against the strand 6 in the above indicated places. The coolant may be water, water-air mixture of steam. By "forced cooling" as described above with reference to Figure 10 it is to be understood that the cooling is so effective that outer parts of the strand 6 by cooling shrinkage deform inner semi-solidified parts of the strand plastically. WHAT WE CLAIM IS:

1. A method of continuous chill casting of steel or other metallic material in the form of a downwardly cast strand wherein during solidification thereof a predetermined elongate region of the cast strand, which comprises solid, semi-solidified and liquid regions and which extends from a casting chill used for the casting to the point where the strand is fully solidified, is caused to become directly or indirectly progressively deformed plastically by the application of a plurality of external deforming means extending along the said region in such a manner that the reduction in crosssection of solidified metallic material caused by cooling shrinkage as it continues to cool is supplemented by the reduction in cross-section of the said solidified material caused by the external deforming means so that the combined reduction is substantially equivalent to or slightly greater than the solidification shrinkage

of adjacent molten metallic material as it changes into the solid state, and any upward or downward transport of melt in the solidifying strand is substantially avoided, thereby preventing or minimising the formation of segregations.

2. A method according to Claim 1, wherein the cross-sectional area of the said region is reduced by 1-10%.

3. A method according to Claim 1, wherein the cross-sectional area of the said region is reduced by 2-6%.

4. A method according to any preceding claim, wherein the predetermined elongate region of the cast strand is caused to become progressively deformed plastically by rolling in one or more steps.

5. A method according to any preceding claim, wherein the casting chill is so shaped that the cast strand has an elongate barrel-shaped cross-section with a pair of opposite longer convex sides, which convex sides are deformed plastically by the action of plane rolls thereon.

6. A method according to any one of Claims 1 to 4, wherein the casting chill is so shaped that the cast strand has an elongate rectangular cross-section, and opposite longer sides of the strand are deformed plastically by the action of rolls thereon, the rolls having a diameter decreasing from the centre to each end.

7. A method according to any one of Claims 1 to 4 wherein the casting chill is so shaped that the cast strand has a square, octagonal or other polygonal or circular cross-section, and opposite edge regions or portions of the strand are deformed plastically by the action of rolls thereon, the rolls having a centrally recessed or concave portion to accommodate the said opposite edges or portions of the strand.

8. A method according to any one of Claims 1 to 3, wherein the predetermined elongate region of the cast strand is caused to become progressively deformed plastically by the action thereon of forging tools reciprocating perpendicularly to the longitudinal direction of the strand and having working surfaces which therebetween define a space tapering in the direction of strand movement.

9. A method according to any one of Claims 1 to 3, wherein the external deforming means constitute forced cooling means which effectively deform inner semi-solidified parts of the strand plastically by cooling shrinkage of outer parts of the strand.

10. A method according to Claim 9, wherein the casting chill is so shaped that the cast strand has an elongate rectangular cross-section, and opposite longer sides of the strand are deformed plastically by the said forced cooling means.

11. A method according to Claim 10, wherein the said forced cooling means are applied to central portions of the said opposite longer sides of the strand.

12. A method according to Claim 9, wherein the casting chill is so shaped that the cast strand has a square or octagonal or other poly gonal or circular cross-section, and the said forced cooling means are applied to opposite edge regions or portions of the strand.

13. A method according to Clairn 1 substantially as herein described with reference to any one of Figures 3-10 of the accompanying drawings.

14. An apparatus for carrying out the method claimed in Claim 1, comprising a casting chill and means for chill casting steel or other metallic material in the form of a downwardly cast strand in such a manner that during solidification thereof a predetermined elongate region of the cast strand is adapted to comprise solid, semi-solidified and liquid regions and to extend from the casting chill to the point where the strand will become fully solidified, and a plurality of external deforming means which are arranged to extend along the said region so as to cause the said region to become directly or indirectly progressively deformed plastically by the application of a plurality of external deforming means extending along the said region in such a manner that the reduction in cross-section of solidification metallic material caused by cooling shrinkage as it continues to cool is supplemented by the reduction in cross-section of the said solidified material caused by the external deforming means so that the combined reduction is substantially equivalent to or slightly greater than the solidification shrinkage of adjacent molten metallic material as it changes into the solid state, and any upward or downward transport of melt in the solidifying strand is substantially avoided, thereby preventing or minimising the formation of segregations.

15. An apparatus according to Claim 14, wherein the external deforming means comprise roll pairs located one after the other in the direction of strand movement, the distance between the two rolls in each pair decreasing with increase in distance from the casting chill.

16. An apparatus according to Claim 14 or Claim 15, wherein the casting chill is so shaped that the cast strand has an elongate barrelshaped cross-section with a pair of opposite longer convex sides, and the external deforming means comprise roll pairs whose rolls have a constant diameter.

17. An apparatus according to Claim 14 or Claim 15 wherein the casting chill is so shaped that the cast strand has an elongate rectangular cross-section, and the external deforming means comprise roll pairs where rolls have a diameter decreasing from the centre to each end.

18. An apparatus according to Claim 14 or Claim 15, wherein the casting chill is so shaped that the cast strand has a square, octagonal or other polygonal or circular cross-section, and the external deforming means comprise roll pairs whose rolls have a centrally recessed or concave portion.

19. An apparatus according to Claim 14, wherein the external deforming means comprise forging tools reciprocating perpendicularly to the longitudinal direction of the strand and having working surfaces which therebetween define a space tapering in the direction of strand movement.

20. An apparatus according to Claim 14, wherein the casting chill is so shaped that the cast strand has an elongate rectangular crosssection, and the external deforming means consitutte forced cooling means comprising a plurality of spray nozzles arranged to spray coolant against the strand so that in operation the forced cooling means adapted to be applied to the opposite longer sides of the strand will effectively deform inner semi-solidified parts of the strand plastically by cooling shrinkage of outer parts of the strand.

21. An apparatus according to Claim 14, wherein the casting chill is so shaped that the cast strand has a square, octagonal or other polygonal or circular cross-section, and the external deforming means constitute forced cooling means comprising a plurality of spray nozzles arranged to spray coolant against the strand so that in operation the forced cooling means adapted to be applied around the strand will effectively deform inner semi-solidified parts of the strand plastically by cooling shrink age of outer parts of the strand.

22. An apparatus for carrying out the method claimed in Claim 1 substantially as herein described with reference to any one of Figures 3-10 of the accompanying drawings.