Twin roll casting

Details Twin roll casting Twin roll casting

1993-03-02

1994-12-13

Bradley, P. Austin

Metal founding

Process

With measuring, testing, inspecting, or condition determination

164480, B22D 1116, B22D 1106

Patent

active

053721802

DESCRIPTION:

BRIEF SUMMARY
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.
An object of the present invention is to provide a twin roll caster having provision for reducing or eliminating sticking of the workpiece.
According to a first aspect of the present invention, in a method of operating a twin roll caster, each roll is driven by independent drive means, either with both rolls rotated at a predetermined speed or with one roll rotated at a predetermined speed and a predetermined torque ratio between the rolls; molten metal is introduced into the gap between the rolls to produce a casting; and characterised in that, at least when sticking of the casting to either of the rolls is detected or is anticipated, the relative speed of the two rolls is adjusted in a manner to reduce the sticking of the workpiece to the roll.
The amount of sticking is dependent on the amount of deformation the material undergoes after solidification is complete (usually referred to as forward slip or extrusion) and this, in turn, is dependent on the material properties and the operating conditions. Casting conditions that result in low values of forward slip encourage sticking. Typically these will include too high a casting speed, too high a casting temperature, too small a tip set back or too little casting tension. By measuring operating parameters, such as, casting temperature, roll temperature, roll coolant temperature or the like, it is sometimes possible to anticipate that sticking is likely to occur. If one or more of the operating parameters start to move outside of an acceptable band of values, sticking of the casting to one of the rolls is likely to occur.
When sticking of the workpiece to one of the rolls is detected, the torque of that roll decreases and, consequently, the power taken by the motor which drives that roll is suddenly decreased and the decrease in power can be detected and used to indicate the onset of sticking. The speed of rotation of the other roll is then decreased and this un-sticks the cast workpiece from the first-mentioned roll.
The speed of rotation of each roll may be modulated about its predetermined speed at substantially 180.degree. out of phase with the other roll. The frequency of modulation may be in the range 0.2 to 2 HZ and the amplitude of modulation may be up to 30% of the predetermined speed. The rolls may be modulated continuously or they may be modulated only when sticking is detected or when sticking is anticipated.
According to a second aspect of the invention, a twin roll caster comp

REFERENCES:
patent: 4054173 (1977-10-01), Hickam
patent: 4565240 (1986-01-01), Shibuya et al.
patent: 4727927 (1988-03-01), Popik
patent: 4735254 (1988-04-01), Itoyama et al.

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