WO1996032215A1

WO1996032215A1 - Continuous roll-casting apparatus

Info

Publication number: WO1996032215A1
Application number: PCT/GB1996/000856
Authority: WO
Inventors: Philip Milroy Thomas; Peter George Grocock
Original assignee: Davy Mckee (Poole) Limited
Priority date: 1995-04-10
Filing date: 1996-04-04
Publication date: 1996-10-17
Also published as: GB9507395D0; GB2314039A; GB9721619D0; EP0830222A1; JPH11503367A

Abstract

In a two-roll caster a refractory nozzle (3) projects into the roll bite on the entry side of the caster. The space between the tip (7) of the nozzle and the adjacent surfaces of the rolls (1, 2) is closed off with resilient refractory material (11) which is mounted on the nozzle and bears against the roll surface.

Description

CONTINUOUS ROLL-CASTING APPARATUS

In a known continuous casting process, molten metal is fed through a refractory nozzle into the bite between a pair of spaced apart counter-rotating cooled rolls. Some of the metal issuing from the nozzle contacts the cooled rolls and forms a solid skin on the surface of each roll. As the rolls rotate, the skins are brought together and undergo a hot reduction as they pass through the roll gap before leaving the rolls as solid strip.

The purpose of the nozzle is to feed the metal uniformly across the width of the roll bite. The nozzle may be made from two parts and the inside of the nozzle contains an array of baffles to bring about uniform flow conditions. Fig. 1 of the accompanying drawings shows, in diagrammatic form, a section through the tip of the nozzle of a twin roll caster. A pair of cooled rolls 1, 2 are spaced apart and a refractory nozzle 3 projects into the bite between the rolls on the entry side of the caster. The passage 4 through the nozzle has a dimension A at the leading edge of the tip of typically 8mm and the thickness B of the upper and lower parts of the nozzle has a typical value of 2mm, so the overall dimension of the nozzle tip i.e. A+2B is typically 12mm and may be in the range 10-15mm.

The nozzle tip is positioned on the centre-line of the roll gap and is pushed into the roll bite until a clearance C of typically 1mm exists between the upper and lower surfaces of the nozzle tip and the top and bottom casting rolls 1,2 respectively. When casting is initiated it is normal for the roll gap to open slightly resulting in an increase in the gap C between the tip and the casting rolls.

Molten metal is forced through the nozzle tip by a metallostatic head which varies according to, inter alia, the hydraulic resistance of the baffling within the tip, and the materials used for the construction of the nozzle.

It is well established that strip produced by the roll casting process can suffer from a number of characteristic defects and that the presence of these defects depends on the precise operating conditions. One defect that is particularly serious is known as surface bleeding and it has been established that the presence of this defect can result in the cast strip cracking during subsequent rolling operations. Surface bleeding is known to occur when casting with low metallostatic heads and the intensity of the problem increases significantly as the casting speed is increased and the thickness of the cast strip is reduced. To overcome the problem of operating with low heads it is relatively easy to increase the metallostatic head but this can result in other problems. For example the metal can be forced to flow back between the tip of the nozzle and the rolls and if this occurs it is likely that the tip will break. Similarly, as the metallostatic head is increased, the width of the strip being cast tends to increase and the edge quality of the strip deteriorates.

It is believed that the problem of surface bleeding and other surface defects such as level lines are due to the changes in shape of the meniscus formed between the nozzle tip and the casting rolls and mechanisms have been proposed to explain the occurrence of these changes. For example, it has been postulated that the meniscus position can vary in a cyclic way and that this is influenced by, inter alia, the nature of the refractory material and variations in the metallostatic head.

Referring now to Figs. 2 (a)-2(c) of the accompanying drawings which show diagra matically part of the tip of the nozzle and the rolls of a twin-roll caster.

The top of the nozzle 3 is shown separated from the adjacent surface of the roll (1). In Fig. 2(a) the separation C between the top edge of the tip and the periphery of roll 1 is shown for a normal operating condition. The meniscus 6 of the molten metal 7 introduced in the bite is shown. If for any reason the periphery of the roll 1 moves slightly away from the top surface of the nozzle as shown in Fig. 2(b) the meniscus 6 of the molten metal 7 is stretched. If the stretching of the meniscus is severe it can rupture and allow molten metal to flow through the ruptured meniscus into the space between the nozzle and the roll as shown in Fig. 2(c).

The problem is exacerbated if the molten metal "wets" the tip of the refractory nozzle because this encourages the molten metal to flow over the surface of the tip of the nozzle.

It is an object of the present invention to provide continuous casting apparatus in which this difficulty is overcome.

According to the present invention, continuous roll- casting apparatus comprises two spaced-apart rolls arranged to be rotated in counter-rotating directions, a refractory feed nozzle for molten metal projecting into the bite between the rolls on the entry side of the caster and gasket means of resilient refractory material mounted on the nozzle and closing off the space between the tip of the nozzle and the adjacent surface of each roll.

Conveniently the nozzle has a tip of substantially rectangular form and separate gaskets of resilient refractory material are mounted on the longer surfaces of the tip of the nozzle so as to contact the respective rolls.

The resilient refractory material may be a fibrous paper material sold under the trade name FIBRE-FRAX which is a relatively soft fibrous material.

Alternatively it can be a graphite based paper material sold under the trade name PAPYEX. This also has the advantage that it applies lubricant to the rolls as it comes into contact with them.

In order that the invention may be more readily understood it will now be described, by way of example only, with reference to Fig. 3 of the accompanying drawings.

In this figure, a pair of counter-rotating rolls 1,2 are arranged one above the other to define a gap between them. Each casting roll may be backed-up with an additional roll. A refractory nozzle 3 having a generally rectangular tip outlet 7 is introduced into the bite on the inlet side. Molten metal flows through the nozzle and out through its tip outlet and into the gap where it starts to solidify in contact with the rolls. The solidified material is rolled by the rolls as it passes through the narrowest part of the gap to form a strip 9.

In order to take up the clearance between the nozzle tip 7 and the casting rolls (1,2) so that the caster can be operated with increased metallostatic heads, a refractory material serving as a gasket 11 is attached to each of the upper and lower surfaces of the nozzle tip and positioned so that the leading edge of the gasket corresponds substantially with the leading edge of the tip as shown in Fig. 3 of the drawings. The resilient material contacts the rolls and this allows higher metallostatic heads to be used without the risk of flooding metal into the space between the upper and lower surface of the tip and the casting rolls and will ensure that stretching and subsequent rupture of the meniscus do not occur. This results in a uniform contact length and stable meniscus.

Further improvements could be achieved by the use of non-wetting refractory materials.

If a meniscus with a consistent shape can be produced cyclic defects such as level lines can be eliminated.

Claims

Claims :

1. A roll caster comprising two spaced apart rolls (1,2) arranged to be rotated in counter-rotating directions and a refractory feed nozzle (3) for molten metal projecting into the bite between the rolls on the entry side of the caster; characterised in that gasket means (11) of resilient refractory material is mounted on the nozzle to close off the space between the tip (7) of the nozzle and the adjacent surface of each roll (1,2).

2. A roll caster as claimed in claim 1 in which the tip of the nozzle is of generally rectangular form and characterised in that a gasket of resilient refractory material (11) is mounted on each of the longer sides of the tip (7).

3. A roll caster as claimed in claim 1 or 2 in which the gasket means is of fibrous paper refractory material.

4. A roll caster as claimed in claim 1 or 2 in which the gasket means is a fibrous refractory material containing graphite.