In the continuous casting of steel slabs it is usual to cast into a water cooled mould having a generally rectangular cross section at the mould exit. Sometimes simple chamfers are added at the corners and sometimes for thinner slabs the mould cross section thickness is enlarged at mid wide side to allow better access for the refractory tube through which molten steel is introduced.

A slight taper is usually introduced in the direction of casting to counteract steel shrinkage during solidification.

If the process is not optimised then cast slabs can suffer from defects of guttering where the slab thickness is reduced immediately adjacent to the slab corners. Additionally slab edge bulging is a frequent defect often leading to internal cracking of the solidified structure.

It is desirable for the fully solidified slab thickness to be reduced in order to minimise the process steps and extent of plant utilised for subsequent rolling of the slab to strip or plate product. It is however desirable to maintain a larger thickness at the mould to provide more favourable liquid steel meniscus conditions which in turn influences the surface quality of the cast slab. One way to reconcile these two desirable features is to reduce the thickness of the strand below the mould but before the slab is fully solidified. In this case the slab contains a liquid core during reduction and the solidified shell of the slab can be relatively thin. Shell buckling and consequent cracks in the solidifying structure is a significant risk in applying this type of reduction.

It is an object of the present invention to provide a method of producing a metal slab which includes continuously casting a metal strand which produces a slab in which shell buckling is at least reduced if not completely overcome.

According to a first aspect of the invention a method of producing a metal slab comprises

continuously casting a metal strand in a continuous casting mould, said strand having a first pair of faces which are flat and are substantially parallel to each other and a second pair of faces each of which includes a portion of concave form, and before the strand is fully solidified, applying rolling forces to said first pair of faces to reduce the thickness of the strand to the thickness of the required slab and to cause the concave portions of the second pair of faces to distort outwardly so that the second pair of faces are substantially flat and mutually parallel.

It is convenient for each of the faces of the second pair of faces to have its portion of concave form to be flanked on either side by a chamfered corner portion. After the application of the rolling forces to the first pair of faces, the concave portions and the chamfered portions of the faces of the second pair distort outwardly so that the second pair of faces are substantially flat, mutually parallel and are at 90° to the first pair of faces.

According to a second aspect of the present invention a continuous casting mould has a mould passage which when the mould is in use produces a metal strand having a first pair of faces which are flat and are substantially parallel to each other and a second pair of faces each of which includes a portion of concave form.

Conveniently the mould passage is shaped so that each of the faces of the second pair has its portion of concave form flanked on either side by a chamfered corner portion.

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: Figure 1 is a plan view of a continuous casting mould; Figure 2 is a perspective view of part of the mould of Figure 1, and Figures 3a-3c show successive changes in the cross section of a metal slab produced by the method of the invention.

A mould (Fig 1) for the continuous casting of steel slabs comprises four cooled copper

plates forming a cavity for the primary shell solidification. Two broad face copper plates (1) are clamped against two end face copper plates (2) to form a cavity in which the initial slab solidified shell shape is formed prior to withdrawal continuously from the mould. The mould will be of a length sufficient to ensure adequate shell thickness at the exit for the required slab withdrawal speed. Slab width change can be carried out during casting by moving the end plates (2) by drives (3) within the confines of the broad face plates (1).

The copper mould end plates (2) have a profiled shape consisting of triangular portions (4) which form an edge chamfer with the broad face plates, and a convex curved face (5) having a radius of curvature proportional to the slab thickness, both of which will present a cooling surface to the liquid metal and provide further shell support and cooling in the mould.

The edge chamfers will continue down the length of the mould end plate whereas the end face curvature may vary down the length.

Figure 2 shows the chamfered/profiled copper mould end plate (2). The plate will be cooled by water which is passed along either slots or holes (6) in the copper back face.

The profiled shape of the end plate may be either as machined copper or coated for wear resistance.

The mould end plate edge chamfers (4) which produce chamfered corners on the newly formed slab shell in the continuous slab casting mould and the mould end plate curved face (5) will give a concave shape to the cast slab edge which will eliminate the need for below mould edge support.

The cross-section of the strand as it emerges from the mould is shown at Figure 3a. It can be seen that it comprises a shell (7) surrounding a non-solidified core (8). The shell has a first pair of flat substantially parallel faces (9) and a second pair of faces (10) each of which includes a concave portion (11) flanked by a chamfered corner (12) at each side where it adjoins

the faces (9).

The mould end plate curved face (5) gives a concave shape to the cast slab edge as it leaves the mould which will reduce internal corner cracks normally associated with slab edge bulging, reduce gutter on the slab broad faces giving improved slab quality during finish rolling while also retaining the ability to slab width change during casting.

After leaving the mould the shell (7), before becoming fully solidified, is subject to reduction in thickness by rolling applied to the faces (9). This may be brought about by a progressive reduction in the gap in the support roller path below the mould. A reduction in the cross-section is shown in Figure 3b.

As the section is reduced then the previously solidified edges rotate until the edge face is approximately flat and the chamfers are flattened in proportion to the amount of reduction. See Fig. 3c.

The mould end plate curved face (5) together with chamfers (4) will give a slab edge profile such that during reduction of thickness with a liquid core immediately after the mould, the slab comers will rotate thus minimising internal and external strain and limiting final convexity of edge during heavy reduction of 5-40 per cent.

With a lighter reduction however, the concave portions will rotate to give new flat edge faces, which retain some corner chamfer.