WO1995030500A1

WO1995030500A1 - Continuous casting moulds

Info

Publication number: WO1995030500A1
Application number: PCT/GB1995/001032
Authority: WO
Inventors: Christopher John Treadgold; Ashok Kumar
Original assignee: British Steel Plc
Priority date: 1994-05-07
Filing date: 1995-05-05
Publication date: 1995-11-16
Also published as: GB9409183D0

Abstract

A continuous casting mould (1) for use in continuous casting apparatus for the continuous casting of metal slab, comprises a narrow rectangular mould (1) in plan view with a generally centrally disposed submerged entry nozzle (5), in which there is provided a float (8) in each elongate half of the mould symmetrically positioned with respect to the submerged entry nozzle.

Description

CONTINUOUS CASTING MOULDS

This invention relates to the continuous casting of steel or other metals and especially to the control of the flow patterns of the liquid metal in the continuous casting mould when casting slab.

Continuous casting of steel and other metals is a well known process and in principle consists of the teeming (or pouring) of molten steel into a mould which is open at the bottom. The mould is cooled and as the liquid steel passes down through the mould the outside is cooled by contact with the cooled mould and forms a skin allowing a cooling continuous slab of metal to be drawn out of the bottom of the mould.

It is known that the flow patterns of the steel in the mould are important in determining the quality of the continuously cast slab. Defects such as blisters, slivers and black lines in hot and cold rolled strip can be caused by the entrapment of surface flux. Longitudinal cracking of slabs, uneven shell growth and in extreme cases breakouts can be caused by asymmetry and instabilities in the bulk and surface flows in the mould.

The submerged Entry Nozzle (SEN) by which the steel is teemed into the mould is a principle means of controlling these defects in conjunction with tight control of the level of the liquid steel in the mould. Generally in slab production the SEN is symmetrically placed within a narrow rectangular-section mould so as to define two halves of the mould, one on each elongate side of the centrally disposed SEN. The preferred flow pattern is symmetrical and recirculating with upper and lower recirculating zones in each half of the mould, together with a flow towards the SEN on the surface.

Investigation show that in many cases this flow pattern is not achieved. It has been found that in practice the flow patterns are often asymmetrical in the elongate halves of the mould and there is a tight circulation in one half of the mould and loose circulation in the other. This leads to surface flows which travel past the SEN and the generation of undesirable von Karman vortices such as are found when fluids flow past a circular obstruction.

It is an object of the present invention to provide means encouraging symmetrical flow patterns in such circumstances. According to the present invention a continuous casting mould for use in continuous casting apparatus for the continuous casting of metal slab comprises a narrow rectangular mould in plan view with a generally centrally disposed submerged entry nozzle wherein there is provided a float in each elongate half of the mould symmetrically positioned with respect to the submerged entry nozzle.

The invention is especially but not solely applicable to the production of steel slab.

The floats may be positioned between 40% and 60% of the distances from the SEN centre line to the end plates of the mould and they may have their long axes aligned with the centre line of the narrow mould. They may be constrained so that they move only small distances in the horizontal plane and may be either fixed in the vertical plane or may be moveable in the vertical plane. They may be made from a refractory material having between 40% and 60% of the density of the liquid metal being cast. The width of the floats may be between 20% and 50% of the width of the narrow mould. The floats may have a length of between 1 and 4 times their width.

In order that the invention may be more readily understood one embodiment thereof now be described with reference to the accompanying drawings in which:-

Figure 1 is a plan of a continuous casting mould according to the invention; and

Figure 2 is a section on the long axis of the mould of Figure 1. Turning now to the Figures, in which the same reference numbers have been used in each, there is shown a continuous casting mould for the casting of steel slabs. The mould is an elongate rectangular shape having two short end walls 2 and two long side walls 3. The steel passes down through the body of the mould which is cooled by means not shown and exits (not shown) as a solidifying slab from the bottom 4 of the mould. The process of continuous casting is well known and will not here be further described. The steel enters through a submerged entry nozzle (SEN) 5 and exits from the nozzle into the mould along paths shown generally by the arrows 6. Clearly a circulation is adduced into the steel in the mould and it has been found, as described above, that a symmetrical circulation as shown by the arrows 7 contributes to a good quality of steel product. Unfortunately it has been found that in practical situations these flow patterns are not achieved, and in particular, as been explained above, surface currents on the top surface of the steel are found which produces waves on the surface and von Karman vortices as the waves flow past the SEN. It will be remembered that in continuous casting the surface of the steel is covered by a flux, not shown in the diagrams, and the waves and vortices on the surface can easily entrap this flux into the steel leading to defects in the solid slab produced.

The invention consists of the provision of two refractory floats placed as shown at 8. These floats are made of an appropriate refractory material, preferably having a density of between 40% and 60% of that of liquid steel. They have the function of damping out the surface waves and movement of the steel, and have been found to contribute greatly to the maintenance of the desired symmetrical flow pattern within the liquid steel. The distance W in Figure 1 is the half width of the mould and the distance X is the distance of the centre of the float from the centre of the SEN which is preferably 40% to 60% of W. The preferred length of the floats in the direction of the long axis of the mould is between 1 and 4 time their width, and float widths of between 20% and 50% of the mould width 10 are considered to be most useful. Too narrow a float will not provide sufficient damping of the flow and too wide a float could lead to problems in bridging between the float and the walls of the mould.

The floats may be fixed and unable to move in any direction, but this is not the most preferred arrangement and in the arrangement illustrated in Figure 2 means are provided to restrain the floats. This means comprises a simple hinge bar connecting the float to a pivot point on the mould. This arrangement allows the float to move up and down with the level of the steel but constrains it in the horizontal plane. The linkage 9 must be designed so as to minimise the horizontal movement of the float as it move up and down, and it is considered that a maximum horizontal movement of 1 to 2 mm is best. Other linkages and arrangements may be devised to maintain the floats in their positions.

Claims

1. A continuous casting mould for use in continuous casting apparatus for the continuous casting of metal slab, comprising a narrow rectangular mould in plan view with a generally central submerged entry nozzle, wherein there is provided a float in each elongate half of the mould symmetrically positioned with respect to the submerged entry nozzle.

2. A mould as claimed in Claim 1 arranged for the continuous casting of steel slab.

3. A mould as claimed in Claim 1 or 2 wherein the floats are positioned within the range 40% to 60% of the distance on the long axis of the mould from the centre line of the submerged entry nozzle to the end plates of the mould.

4. A mould as claimed in Claim 1, 2 or 3 wherein the floats are disposed with their long axes aligned with the centre line of the mould.

5. A mould as claimed in any one of the preceding Claims wherein the floats are constrained in use such as to be capable of only small movement in the horizontal plane.

6. A mould as claimed in Claim 4 wherein the floats are movable in the vertical plane.

7. A mould as claimed in any one of the preceding Claims wherein the floats are fabricated from a refractory material having a density of between 40% and 60% of the density of the liquid metal being cast.

8. A mould as claimed in any one of the preceding Claims wherein the floats are within the range of 20% and 50% of the width of the mould.

9. A mould as claimed in Claim 8 wherein the floats have a length in the range of 1 to 4 times their width.

10. A mould as claimed in any one of the preceding Claims wherein each of the floats is generally brick-shaped.