CONTINUOUS CASTING MOULD

Abstract: A continuous casting mould (4) for casting thin slabs has, at its inlet end, a mould cavity which has generally rectangular side regions (7) joined by a central pouring region (8). At least part of the walls of the pouring region are of arcuate form and over at least part of the depth of the mould the radii of curvature of the arcuate parts progressively increase.

Description: CONTINUOUS CASTING MOULD This invention relates to continuous casting of metal and in particular to a continuous casting mould for casting workpieces in the form of thin slabs. European Patent Application No. A-0149734 discloses a continuous casting mould of the type to which the present invention relates. This publication discloses a continuous casting mould having a mould cavity which extends be:wee.n the inlet and outlet ends of the mould. At the inlet end of the mould, the cavity, in its widthwise direction, comprises two side regions of generally rectangular form connected by an enlarged central funnel-shaped pouring region into which, in use, the feed tube extends. The pouring region gradually reduces in size along the depth of the mould until at a position above the outlet end of the cavity the pouring regicn disappears. With such a mould, tensile strains are applied to the solid/liquid interface of the inside of the metal shell as it passes down through the mould. These tensile strains can be detrimental when their πagnitude is large enough to open the grain boundaries which allows interdendritic penetration by the liquid solute which can cause detrimental segregation. These strains may also create internal cracks. An object of e present invention is to provide a continuous casting mould in which these disadvantages are overcome. According to the present invention, there is provided a continuous casting mould having a cavity which extends between the inlet and outlet ends of the mould and where at the inlet end of the mould, the cavity, in its widthwise direction, comprises two side-regions of generally rectangular form connected by an enlarged central pouring region, characterised in that said pouring region extends to the outlet end of the mould and along its length it is of constant width; and the opposite walls of the pouring region are entirely of arcuate form and over at least that part of the length of the pouring region which extends to the outlet end of the mould the radii of curvature of the arcuate walls progressively increase. A containment zone may be positioned beneath the casting mould and the pouring region may extend into the containment zone. In order that the invention may be more readily understood, it will now be described, by way of example only, with reference to the accompanying drawings, in which:- Figures 1 and 2 are diagrammatic front and sectional side elevations, respectively, of a continuous casting mould in accordance with one embodiment of the invention; Figures 3 and 4 are diagrammatic front and sectional side elevations, respectively, of a continuous casting mould in accordance with a second embodiment of the invention; and Figures 5 and 6 are diagrammatic front and sectional side elevations, respectively, of a continuous casting mould in accordance with a third embodiment of the invention. Referring now to the figures, a continuous casting mould for casting thin metal slabs, or thick strip, is indicated by reference numeral 1. The mould is conveniently of copper alloy and it is cooled in a conventional manner by means (not shown). In each embodiment, a mould cavity 2 extends from the inlet to the outlet end of the mould. There may be additional cavities (not shown) in the mould. The mould is oscillatable in the direction of the length of the mould cavity by means (not shown). In use, the mould is positioned above a containment zone 3, which may consist of grids and rollers 4 and which supports the cast slab whilst it is cooled and the shell of the slab thickens. A refractory feed tube 5 extends into the inlet end of the mould cavity and molten metal passes through the tube into the mould. The feed tube may be of the form described in our co-pending Application of even date. In each embodiment, at the inlet end of the mould, the cavity 2 in its widthwise direction comprises two side regions 7 which are rectangular and a central portion 8 which is larger to form a pouring region which accommodates the ceramic feed tube 5 so that adequate clearance exists between the mould walls and the outside of the tube 5. At least part of the enlarged central pouring region of the mould cavity constitutes a deformation area 9 to deform that part of the shell formed in it as the shell is moved in the direction of discharge. The deformation area is shaded in the figures. In the Figures 1 and 2 and Figures 5 and 6 embodiments it extends to a position above the bottom outlet end of the mould. In the Figures 3 and 4 embodiment it extends to the " outlet end of the mould. The deformation area may commence at the inlet end or at a position inwardly, of the inlet end. Referring now to Figures 1 and 2, the centre portion 8 has walls which are of arcuate form. The arcs are of ever increasing radius R which increase to infinity at depth L. Over the distance 1-^, from the top of the mould the radius remains as R but at 1 2 the radius R2 is such that R 2 R j _ whilst the length of the chord C remains constant. In this arrangement the mould discharge shape is a rectangle and the strand issuing from the mould is a rectangular solidified shell with a liquid core as shown by reference numeral 10. In such system the solid/liquid interface of the strand shell is basically subjected to continuous compressive stresses at the vertical edges of the shell deformation area 9. As the shell is cooled by the mould wall it contracts setting up a stress in the outer layer. Along the bottom horizontal edge of area 9, where the radius R becomes infinite, the stresses remain compressive as the transition from curved to flat is a gradual continuous change. The arrangement shown in Figures 3 and 4 has a shell deformation area 9 which extends into the containment zone 3. The containment zone may comprise a series of rollers 41, as shown, or support grids and rollers any of which are profiled to the radius required, i.e. R^, R 2 , R3 , etc., where 3 R 2 R j . The metal flowing into the mould may be in liquid or solid plus liquid state. When the metal is in the solid plus liquid state, which is known as slurry, the structure will be finer with smaller grains than casting with temperature above the liquids. A structure with such small grain size will require less mechanical working to give a suitable structure in the final product as well as eliminating the possibility of interdendritic solutes causing detrimental segregation. The forces applied to the shell are basically compressive resulting from a unifor ally distributed load on an arch whose ends are constrained at the points where the ever increasing radius meets the planar narrow parallel ends. The comprehensive stresses are lowered as the radius of curvative are increased and the rate of change is lowered. Thus, from the meniscus, the shell in the centre portion 8 of the cavity will have two opposite vertical arches which are continuously increasing in radius over a width which does not reduce as the strand passes through the caster. The narrow face support adjacent to the deformation zone will contain the forces resulting from the shell deformation. In the arrangement shown in Figures 5 and 6, the centre portion 8 of the cavity is made up of opposite walls each of which has arcuate end portions 12 joined by straight portions 13. The radii of curvature of the arcuate portions 12 increase along the length of the mould cavity and become infinity at the lower end of the region 9. The cross-section of the casting as it leaves the outlet end of the mould is shown by reference numeral 15. Instead of totally deforming the shell to a rectangular flat shape, the strand may be allowed to totally solidify with a thicker centre section prior to either full or final deformation into a rectangular flat section equal to or less than the thickness of the narrow mould ends by rolling or forging prior to being cut into usable lengths. The strand support below the mould may initially be configured to follow the orientation of the mould or it may be configured so that the direction of discharge is changed, e.g., the mould may be vertical and the strand support may also be vertical or it may be curved immediately below or at a discrete distance below the meniscus either inside or outside the mould. The feed tube can be manufactured from or.e of a number of refractory materials. These materials have different thermal characteristics which will affect the temperature of the liquid metal adjacent to the tube and the melting of the mould lubricant, hence the surface quality of the cast product. The feed tube can also be of composite construction with different materials along its length or through its thickness. In some instances this can be a detrimental effect, especially where melting of the lubricant is inhibited or where the bridging between shell and tube may occur. In order to minimise/overcome this, the mould material is chosen such that the thermal conductivity at the area adjacent to the ceramic feed tube is different to give a different heat removal rate to other parts of the mould to enable the performance of the mould powder to remain relatively constant all around and down the mould. For example, an insert 16 (Figure 1) may be placed in the centre of each of the longer walls of the mould cavity, the length of the inserts being approximately equal to the length of the feed tube. The insert has a lower thermal conductivity than the remaining parts of the mould and this ensures that the metal is hotter and the shell remains thinner in the vicinity of the feed tube. The insert may be a matrix of metal such as copper with a ceramic such as silicon nitride or boron nitride impregnated in it. The widthwise dimension of the mould cavity may be adjustable. For example, the walls of the mould which define the ends of the side regions 7 of the cavity may be movable towards and away from each other to adjust the length of the side regions and hence the width of the strand cast in the mould.

Claims: Cl aims : (1) A continuous casting mould having a cavity which extends between the inlet and outlet ends of the mould and where at the inlet end of the mould, the cavity, in its widthwise direction, comprises two side-regions of generally rectangular form connected by an enlarged central pouring region, characterised in that said pouring region extends to the outlet end of the mould and along its length it is of constant width; and the opposite walls of the pouring region are entirely of arcuate form and over at least that part of the length of the pouring region which extends to the outlet end of the mould the radii of curvature of the arcuate walls progressively increase. 2. A continuous casting mould as claimed in claim 1, in which a containment zone having inlet and outlet ends is positioned with its inlet end at the outlet end of the mould and the pouring region extends into the containment zone. 3. A continuous casting mould as claimed in claim 2 , in which the radii of curvature of the walls of the pouring region increase to infinity at a position above the outlet end of the containment zone so that the pouring region does not extend to the outlet end of the containment zone. 4. A continuous casting mould as claimed in any preceding claim in which the radii of curvature of the arcuate walls progressively increase from a position spaced inwardly from the inlet end of the mould. 5. A continuous casting mould as claimed in any preceding claim including means for oscillating the mould in the direction parallel to the length of the mould cavity. 6. A continuous casting mould as claimed in any preceding claim, in which the opposite walls of the pouring region, at least at the inlet end of the mould, include inserts of a material having a lower thermal conductivity than the material from which the remaining parts of the mould are formed. 7. A continuous casting mould as claimed in claim 6, in which the inserts are of a copper matrix impregnated with silicon nitride or boron nitride. 8. A continuous casting mould as claimed in any preceding claim in which the widthwise dimension of the mould cavity is adjustable.

Inventors: PERRY ROBERT MAIDENS, REYNOLDS TIMOTHY, PERRY, ROBERT, MAIDENS, REYNOLDS, TIMOTHY
Applicants: DAVY DISTINGTON LTD, DAVY (DISTINGTON) LIMITED
Application year: 1989
Main classification: B22D11
IPC codes: B22D11, B22D41