There have been various recent proposas to improve the design of impact pads, particularly with a view to eliminating, or at least reducing, surface turbulence in a continuous casting tundish, to minimise slag entrainment within the liquid steel bath in the tundish, to prevent break-up of tundish flux cover and reoxidation of the liquid steel bath and to ensure a proper flow path. These designs, essentially, have included a base for the pad and upstanding walls around the perimeter of the base to define an opening into which the molten metal stream is received.

Thus, for example, in U. S. patent no. 5169591 is disclosed an impact pad having a base to receive an incoming ladle stream and one or more sidewalls extending upwardly along the periphery of the base. Each sidewall has an inner surface having an undercut portion facing the incoming ladle stream and extending along the length of the inner surface, thereby providing a shaped surface to receive and reverse the direction of fluid flow generated by the incoming ladle stream.

U. S. patent no. 5358551 discloses an impact pad having a bottom impact surface and an endless sidewall extending upwardly from the bottom impact surface and defining an opening into which molten metal is poured. The inner surface of the endless sidewall includes an annular portion which extends inwardly and upwardly towards the opening. Preferably the inner surface of the sidewall curves continuously from the bottom impact surface to a vertical wall defining the opening.

In our International patent application No. PCT/GB97/00899 we have described and claimed an improved impact pad which results in improved flow characteristics of molten steel poured into a tundish. Darticularlv for a longitudinally-extending tundish in which the steel input zone and steel outlet zone (s) are towards opposite ends of the tundish.

Thus, the invention of our aforesaid International application provides a tundish impact pad comprising a body of refractory material capable of withstanding contact with molten steel, the body comprising a base having an impact surface for molten steel, an outer sidewall extending upwardly from the impact surface, the outer sidewall extending around the base to completely enclose it, an annular body portion connected to the sidewall and providing a top surface substantially parallel to the impact surface and defining an opening into which molten steel can be poured, the lower face of the annular body portion and the inner face of the sidewall defining a recess having an undercut portion extending continuously around and above the impact surface, wherein the improvement comprises a first portion of the top surface being at a lower level than the remainder of the top surface and the recess beneath the first portion of the top surface being of smaller cross-section than the remainder of the recess.

Whereas this improved impact pad has been found to be particularly advantageous in the above-mentioned longitudinally-extending type of tundish during normal use of the tundish, we have now found that there is room for even further improvement. As is explained in our aforesaid International patent application, when the impact pad is positioned in a tundish having an inlet zone adjacent one end thereof, the pad should be positioned with its end containing a portion of its top surface at a lower level facing the inlet end wall of the tundish. After start up we have found this arrangement to be most advantageous. However, we have now found that during start up there is still further room to improve the pad design.

Accordingly in one aspect the invention provides a tundish impact pad comprising a body of refractory material capable of withstanding contact with molten steel, the body comprising a base having an impact surface for molten steel, an outer sidewall extending upwardly from the impact surface, the outer sidewall extending around the base to completely enclose it, an annular body portion connected to the sidewall and providing a top surface substantially parallel to the impact surface and defining an opening into which molten steel can be poured, the lower face of the annular body portion and the inner face of the sidewall defining a recess having an undercut portion extending continuously around and above the impact surface, wherein a first portion of the top surface is at a lower level than the remainder of the top surface and the recess beneath the first portion of the top surface is of smaller cross-section than the remainder of the recess, a temporary shield of material destructible by the motten meta) being positioned on or adjacent to the lower surface portion to reduce or eliminate its height loss relative to the remainder of the top surface.

The shield may be made, for example, of steel or of a refractory material that will be eroded after a pre-determined exposure to the molten metal.

The shield may be attached to the impact pad by any convenient means. For example, particularly for a steel shield, the impact pad may be made with a slot to receive an edge of the shield and the shield may be cemented into the slot using a suitable adhesive, e. g. a refractory cement. Thus the attachment is made after the impact pad has been fired. Alternatively, for example, particularly for a shield made of refractory material, the shield may be mounted in the desired position in a tool in which the impact pad is to be cast so that the pad may be cast in-situ around an edge of the shield.

, At start up, when molten metal is first poured into the tundish, and the risk of undesirable splashing is at its greatest, the shield prevents splashing over what would otherwise be the low sidewall of the pad. As the tundish fills up with molten metal, the splashing diminishes and is considerably reduced, if not completely eliminated, when the tundish is full and steady state operation commences. By this time the temporary shield will have melted or eroded away leaving the impact pad to operate in its advantaaeous form with the lower top surface portion at one side.

The invention is particularly advantageous in a single strand caster if the position of the inlet for molten metal constrains the impact pad to be positioned away from the adjacent end wall of the tundish. (It will be appreciated that, if the pad can be positioned almost in contact with the tundish wall, the wall itself may act as a splash preventer thereby possibly obviating the need for the pad of this invention.) Preferably the lower face of the annular body portion and the inner face of the sidewall of the pad provide a continuous curved surface which, at its lower end, merges into the impact surface of the base thereby providing a concave recess around the base, the recess having two discontinuities or steps, i. e. one at each end of the smaller recess portion.

The impact pad is preferably four-sided in plan and, in a particularly preferred embodiment, it has one pair of parallel sides, (defined by the outer surface of the sidewall) i. e. it is wedge-shaped. In this latter embodiment, the longer parallel side preferably includes the first portion of the top surface, i. e. includes the lower top surface portion where the temporary shield is to be fitted, and the smaller recess.

Alternatively, the pad may be, for example, rectangular in plan view.

The smaller recess is preferably not coterminous with the lower top surface portion and it is especially preferred that the smaller recess portion does not extend as far as the ends of the lower top surface portion, i. e. the smaller recess portion is not as long as the lower top surface. Thus, in the preferred wedge-shaped pad construction, the lower top surface portion stops a little short of each end of that sidewall to provide a step up to a top surface level with the remainder of the surface extending around the other sides of the pad and the smaller recess portion extends underneath the lower top surface portion sufficiently that each of its ends overlaps the recess in its respective adjacent side of the pad but stops short of the aforesaid step.

In this embodiment, therefore, there is in effect a cut-out portion in the top surface along one of the four sides of the pad.

As indicated above, the impact pad of the invention is particularly intended for use with a longitudinally-extending tundish having a steel inlet zone towards the end opposite to the outlet zone (s).

Accordingly, in another aspect the invention provides a tundish having an inlet zone adjacent a first end thereof and an outlet zone adjacent the opposite end thereof, an impact pad positioned on the floor of the tundish in the inlet zone, the impact pad comprising a body of refractory material capable of withstanding contact with molten steel, the body comprising a base having an impact surface for molten steel, an outer sidewall extending upwardly from the impact surface, the outer sidewall extending around the base to completely enclose it, an annular body portion connected to the sidewall and providing a top surface substantially parallel to the impact surface and defining an opening into which molten steel can be poured, the lower face of the annular body portion and the inner face of the sidewall defining a recess having an undercut portion extending continuously around and above the impact surface, a first portion of the top surface being at a lower level than the remainder of the top surface and the recess beneath the first portion of the top surface being of smaller cross-section than the remainder of the recess and a temporary shield being positioned on or adjacent the first portion of the top surface, wherein the pad is positioned with its first portion of the top surface nearest to the first end of the tundish.

In the particularly preferred embodiment, therefore, where the impact pad is of wedge-shape, the longer parallel side of the pad is positioned closest to the first end of the tundish.

The impact pads of the invention have been found to be particularly useful in reducing surface turbulence in tundishes of the type described above and are equally useful in tundishes which are generally rectangular in plan view and tundishes which are generally of wedge shape, i. e. with two parallel sides, in plan view. Moreover, the flow patterns of the molten metal within the tundish can be improved over those resulting from use of conventional impact pads.

The impact pads may be formed from a castable refractory composition capable of withstanding continuous contact with molten metal, in particular molten steel such as is used in continuous casting operations. Usually a standard medium-to-high alumina refractory with an alumina content in the range of about 55% to 85% by weight is desirable. Where a basic refractory is preferred because of steel chemistry, it is preferred that a magnesia-based refractory composition be utilise, with MgO in the range of about 58% to 93% by weight.

The invention is now described by way of example only with reference to the accompanying drawings in which : Figure 1 is a plan view of an impact pad of the invention ; Figure 2 is a section on linge 11 - li of Figure 1 ; Figure 3 is a view in the direction of arrow A of Figure 1 ; Figure 4 is a plan view of one form of tundish of the invention containing an impact pad having a shield in place ; Figure 5 is a plan view of another form of tundish of the invention similarly containing an impact pad having a shield in place ; Figure 6 is a section on) ine V) - VI of Figure 5 ; Figure 6A is an entarged view of a portion of Figure 6 ; Figure 7 is a similar view to Figure 4 showing flow of molten metal after the shield has been destroyed ; Figure 8 is a similar view to Figure 5 showing flow of molten metal after the shield has been destroyed ; Figure 9 is a section on line IX - IX of Figure 8 ; and Figure 9A is an enlarged view of a portion of Figure 9.

In Figures 1 to 3, a tundish impact pad 10 is of wedge-shaped plan form having two parallel sides 11 and 12, of which side 11 is longer, and two non-parallel sides 13 and 14 of equal length.

The pad has a base 15 and an impact surface 16 to receive a stream of molten steel. An outer sidewall 17 extends upwardly from the impact surface 16 and extends completely around the base, i. e. it provides all four sides 11,12, 13 and 14 of the pad. An annular body portion 18 is connected to the side wall and provides a top surface 19 substantially parallel to impact surface 16 and defines an opening 20 into which molten steel can be poured.

The lower face of the annular body portion 18 and the inner face of the sidewall 17 define a recess 21 having an undercut portion 22 which extends around and above impact surface 16.

The longer side 11 of the pad has a sidewall 17A connected to an annular body portion 18A having top surface 19A at a lower level than the remainder of the tnn surface 19 The) nwer face of annular bodv oortion 1 RA anri thP innPr fanP nf sidewall 17A define a recess 21A having an undercut portion 22A. Recess 21A is of sma ! ! er cross-section than recess 21.

As shown most clearly in Figure 1, the lower top surface 19A does not extend for the full length of side 11 of the pad. It stops a little short of each end of that side to provide a step 23 (Figure 2) up to a corner top surface portion 19C which is level with surface 19 extending around the other sides of the pad.

Recess 21A extends beneath top surface 19A but stops short of step 19C at each end of top surface 19A. It does, however, extend sufficiently to overlap with recess 21 in the sidewalls 17 of sides 13 and 14 of the pad.

An L-shaped steel shield 24 is attached to run along the inner edge of top surface 19A so that one arm 24A of the shield extends substantially vertically, i. e. at a right angle to base 15, and the other arm 24B extends inwardly and horizontally, i. e. parallel to the base. Thus the shield effectively increases the height of wall 17A to that of the remainder of the surrounding wall 17 and arm 24B provides a temporary top surface equivalent to surface 19.

Figure 4 shows the impact pad 10 of Figures 1 to 3 in position on the floor 31 of a tundish 30. The tundish is longitudinally-extending and wedge shape in plan and has an inlet region 32 adjacent its narrow end 30A and two outlets 33 towards its wider end 30B. Impact pad 10 is positioned in the inlet region 32 with its longer side 11 nearest narrow end 30A of the tundish.

Figures 5 and 6 show a similar arrangement for a tundish 34 of rectangular plan. This tundish has an inlet region 35 and an outlet 36 towards its opposite ends 34A and 34B respectively. Impact pad 10 is again positioned on floor 37 in the inlet region with its longer side 11 facing end wall 34A of the tundish.

The tundish in Figures 6 and 9 is shown to have a pouring tube 35A to direct incoming steel into the impact pad 10 and a dam 38, which may be provided where necessary and as is conventionally known.

Figure 6A shows the impact pad from Figure 6 in greater detail with shield 24 in place and Figure 9A similarly shows the impact pad from Figure 9 in greater detail with shield 24 destroyed.

Steel flowing into impact pad 10 at start up, i. e. when the tundish is empty, flows outwardly towards the sidewalls of the pad after hitting impact surface 16 and is then turned in recesses 21 and 21A to flow out of the pad over surfaces 19 and 24B. As the tundish fills up the shield melts and flow out of the tundish will continue over surface 19 but will now also be over the lower surface 19A. This situation is as shown in Figures 7 and 8. Thus, as indicated by the arrows in Figures 7 and 8, flow towards walls 30A and 34A is over lower top surface 19A.

This arrangement has been found to reduced markedly the surface turbulence created in the tundish and the improved flow paths created result in cleaner steel production.