TWIN ROLL CASTING

Abstract: A twin roll caster has its two rolls (5) rotated with their axes of rotation in parallel planes but inclined to each other. In this way, sticking of the casting to the rolls is reduced or eliminated.

Description: TWIN ROLL CASTING This invention relates to the casting of metal, particularly light metal alloys, by the twin roll casting process. In this known process, liquid metal is introduced into the bite between two cooled rotating rolls. Where the liquid metal comes into contact with the rolls, a skin is formed and this skin, which rapidly thickens, undergoes hot working before finally emerging from between the two rolls in the form of a strip. During the hot working of the material, the cast material initially is extruded towards the liquid metal feed because the strip is moving more slowly than the surface of the adjacent rolls. Where the strip is extruded from the roll bite, it leaves the machine at a greater velocity than the peripheral surface of the adjacent rolls and this is known as "forward extrusion". There is a neutral point between the entry side of the molten metal and the exit side of the strip where the movement of the strip is equal to the peripheral speed of the adjacent rolls. It is usual for both of the rolls, which are of the same diameter, to be driven from a common source and thus both roll surfaces have the same peripheral speed. In these circumstances the neutral point is in the same position for each of the two rolls. One of the problems encountered in twin roll casting is sticking of the casting to one or other of the rolls. In general, sticking becomes a greater problem as the thickness of the workpiece being cast is reduced. When casting workpieces of a thickness of, say, less than 2.5 mm, sticking of the casting to one or other of the rolls can become very critical. It is an object of the present invention to provide a twin roll caster in which the problem of sticking is at least reduced, if not entirely eliminated. According to the present invention, a twin roll caster has provision for displacing at least one of the rolls such that the axes of rotation of the rolls remain in parallel planes but are inclined to each other. It is well established in the art of metal rolling that the profile of the material being rolled can be controlled by crossing the mill rolls. However, in the twin roll casting process, the profile of the cast strip is readily controlled by means other than by crossing the rolls. In the twin roll caster of the present invention, roll crossing is used to prevent sticking of the cast strip to either or both of the caster rolls. By crossing the rolls of a twin roll caster, a scrubbing action is generated between the cast strip and the rolls independently of any forward extrusion and a slip force is applied to each surface of the cast strip. This additional force is perpendicular to the direction of movement of the strip in the plane of the strip and serves to generate a sideways scrubbing action between the surface of the strip and the rolls, thus preventing the strip material from sticking to the rolls. It has been found that sticking can be avoided if the forward slip (or extrusion) is greater than typically 5%. This amount of relative movement can be achieved by crossing the caster rolls by approximately 6 degrees. One or both of the rolls can be displaced by means of actuators which act between the chocks at the ends of the roll and the rigid housing of the caster. Although the position of the rolls can be altered once casting has commenced, it is more likely that the roll(s) will be displaced to the crossed position prior to the commencement of casting and held in that position during casting. The angle of crossing of the rolls is dependent on the casting condition, inter alia, the alloy being cast. According to a second aspect of the invention, in a method of operating a twin roll caster, molten metal is introduced into the gap between a pair of cooled rolls, said rolls being rotated with their axes of rotation in parallel planes but inclined to each other. In order that the invention may be more readily understood, it will now be described, by way of example only, with reference to the accompany drawings, in which:- Figure 1 is side elevation of a twin roll caster; Figure 2 is sectional end elevation of the caster shown in Figure 1; Figure 3 is a detail of one end of a roll of the caster; and Figures 4a - 4c indicate the crossing of the rolls of the caster. Referring particularly to Figures 1 and 2, a twin roll caster comprises a pair of spaced apart housings 1, each of which defines a window 3. A pair of rolls 5 are supported at their ends in bearing chock assemblies which are located in the housing windows 3 so that the two rolls are arranged one above the other. Means (not shown) are provided for circulating cooling fluid through the rolls. Roll gap adjustment means 7 are provided in each housing window to raise and lower the upper roll so as to adjust the gap between the two rolls. A nozzle assembly 9 serves to introduce molten metal into the gap between the rolls and the molten metal coming into contact with the cooled rolls forms a pair of skins and these skins, which rapidly thicken. undergo hot working before finally emerging from between the two rolls in the form of a metal strip. The rolls are rotated by drive means 11 which may be a common drive but, preferably, each roll is driven from a separate drive motor, as shown in Figure 1. This enables rolls of different diameters to be used and for the speed of rotation of one roll to be adjusted relative to that of the other, if this becomes necessary. In a conventional twin roll caster, the axes of rotation of the two rolls are in parallel planes and are parallel with each other. Referring now to Figure 3, one of the rolls 5 has its roll end supported in bearings 6 mounted in a thrust bearing cage 8 having a part spherical outer surface. The cage is mounted in a chock assembly 13 which has a width which is less than the width of the window in the housing 1. A pair of fluid operable actuators are located in recesses 19 in the housing 1, on each side of the window. The actuators, when energised, engage the opposite sides of the chock assembly 13. The line of action of each actuator is substantially normal to the axis of rotation of the roll. Although not shown each roll has means to resist end thrust. In order to cross the rolls two modes may be employed. As shown in Figures 4a and 4b, each roll is displaced by an equal lateral movement of the bearing chocks at each end of the roll by operation of the actuators with the lateral movement at one end being in the opposite direction to the lateral movement at the opposite end and, in this way, each roll pivots about its centre. In the arrangement shown in Figure 4c, however, the bearing chock assembly at the right-hand end of, say, the top roll is displaced in one direction by the actuators while the bearing chock at the other end of the roll is not displaced. At the same time, the bearing chock assembly at the left-hand end of the lower roll is displaced by the actuators while the opposite end of the lower roll is not displaced and, in that way, each roll pivots about one end but the two rolls are pivoted about opposite ends. In all embodiments of the invention it is convenient for the rolls to be displaced so that they are inclined to each other at an angle of between 2 and 10 degrees, about 6 degrees being the most usual. Although the actuators can be operated during casting in order to adjust the angle of inclination between the two rolls, it is more usual to incline the rolls prior to casting and to operate the caster with the rolls at a fixed mutual inclination.

Claims: Claims : 1. A twin roll caster characterised in that it has provision for displacing at least one of the rolls such that the axes of rotation of the rolls remain in parallel planes but are inclined to each other. 2. A twin roll caster as claimed in claim 1, wherein each roll is supported at its ends in bearing chock assemblies, and characterised in that the bearing chock assemblies of at least one of the rolls have actuators associated therewith for displacing the bearing chock assemblies relative to a rigid structure forming part of the roll caster. 3. A twin roll caster as claimed in claim 2, characterised in that each bearing chock assembly has two opposed pairs of actuators associated with it. 4. A twin roll caster comprising a housing structure; a pair of fluid cooled rolls rotatably mounted in the structure with the axes of rotation of the rolls in parallel planes; drive means for rotating the rolls; means for adjusting the gap between the rolls; a nozzle assembly for introducing molten metal into the gap between the rolls; characterised in that actuators are provided in association with the bearing chock assemblies of at least one of the rolls to enable the axes of rotation of the rolls to be mutually inclined. 5. A twin roll caster as claimed in claim 4, characterised in that said actuators comprise fluid operable rams acting between the bearing chock assemblies and the housing structure. 6. A method of operating a twin roll caster in which molten metal is introduced into the gap between a pair of cooled rolls, said rolls being rotated with their axes of rotation in parallel planes but inclined to each other. 7. A.method of operating a twin roll caster as claimed in claim 6, characterised in that the axes of rotation of the rolls are inclined to each other in the range 2 - 10 degrees. 8. A method of operating a twin roll caster as claimed in claim 6, characterised in that the axes of rotation of the rolls are inclined to each other by about 6 degrees.

Inventors: GROCOCK PETER GEORGE, THOMAS PHILIP MILROY, GROCOCK, PETER, GEORGE, THOMAS, PHILIP, MILROY
Applicants: DAVY MCKEE POOLE, DAVY MCKEE (POOLE) LIMITED
Application year: 1991
Main classification: B22D11
IPC codes: B21B13, B21B27, B21B31, B22D11