Billet, bloom and slab shapes are given to the molten steel by the casting die in continuous casting and can be compared to a mould made of water-cooled copper in which targeted extraction of heat causes the steel to form an outer skin – at first as delicate as a raw egg. So that the freshly formed strand shell does not adhere to the copper wall during this process, the casting die oscillates along the casting plane. An oscillation mechanism helps with this process. SMS Concast developed an electromotive solution with direct drive in an intensive co-engineering process with Kollmorgen. Condrive is based on winding optimised KBM motors to form a process in continuous casting.
In addition to this, the tensile force resulting from the movement profile must not lead to the just-formed shell being torn up again. If this were the case, molten steel would pour out into the system, leading to immediate interruption of casting and subsequent production outage. As a rule, a sinusoidal oscillation process is employed. What is crucial for this is the time window in which the oscillating, downward-moving, casting die overtakes the steel strand, which is also moving downwards, before the forming unit at the lower peak travels back up. This overtaking process is known as “negative strip time” and defines the time in which the casting powder can lie on outer wall of the steel while being pulled off. The oscillating movement of the casting die requires that the fine waves are formed on the surface of the strand, known as oscillation marks.
Adjustable oscillation heights
The ideal frequency at which the casting die should be oscillating in the continuous casting and, above all, at which amplitude, depends on factors, such as, the quality of the steel, the formulation, the production speed or even the casting die form. The negative strip time typically lies in a time window between 0.08 and 0.18 seconds. “If we don’t have this time, we cannot continue to cast. Then the entire mechanics fail to function, i.e. the friction characteristics between the copper and steel,” explains Kabosch and he likens it to liquid chocolate cake sticking to the baking sheet and, when removed from the tin, it breaks apart. The interplay between friction force or friction coefficient and the ferrostatic pressure, which acts vertically on the inside of the casting die due to the molten steel, is fragile. “At this point, we use the direct drives by Kollmorgen,” explains the SMS Concast developer. The transfer of mechanical processes into a software-based motion control opens up possibilities of effectively implementing new movement curves and being able to flexibly and individually adjust the movement to the differing casting conditions. The advantage is that the frequency, the amplitude and the wave form of the oscillation can be changed. The amplitude is crucial for distance covered within a time window. “Converting a mechanical eccentric shaft for this is only possible during shutdown and takes a long time,” notes Dino Kabosch. Also, compared to hydraulic drives for oscillation, the Condrive scores highly since the actuator solution does not require a hydraulic infrastructure and comes with significantly lower maintenance costs.
Direct drive technology by KBM motors: compact size
The torque motors specially adjusted by Kollmorgen to the needs of this application originate from the KBM kit. In the 118-installation size, the drives deliver torques of 660 Nm and a peak torque of 1000 Nm. The drive is thus, able to precisely get the oscillation bulk, weighing between 3.5 and 7 tons, into oscillation. On the control side, SMS Concast also opts for Kollmorgen–in this case in the design of the S700 servo amplifiers.
What is unique about the production of oscillation is the fact the oscillation is not generated by a revolving eccentric shaft in a rotative continuous operation, but by the recurrent reversing motions of the motor. What may seem simple at first glance, has brought about very demanding challenges for engineering. The constant reversing leads to questions such as, how the heat loss is effectively diverted. Other questions relating to lubrication also emerge.
Since the motor is no longer making complete revolutions, the grease for standard lubricated-for-life bearings is inevitably suppressed by the rolling elements. The effect of the lubrication is thus, no longer guaranteed. Finally, these three basic conditions have contributed towards Kollmorgen equipping the KBM motor with a water-cooling system so that the heat loss can be effectively diverted. For the long-term effective lubrication of the few bearings of the gearless direct drives, stators and rotors are completely enclosed in an oil bath — which ultimately also securely protects the encapsulated unit against metal dust.
Another advantage results from the eradication of the drive – namely with an eye on continuous condition monitoring of the system and processes. By the KBM motor driving, the process without the influence of disruptive gear effects, such as, play or friction, ensure an extremely precise control of processes while also very directly and precisely monitoring the process due to the motor data. It is above all key motor data, such as, power and revolutions, which allows the processes to be precisely analysed – up to and including the incorporation into IoT systems.
Potential for modernisations
Currently, Condrive is being developed into a sought-after series product which, thanks to its simple integration and high availability, is very well suited to the modernisation of existing continuous casting machines. Contracts in Spain, India and China mark the success of the solution with the actuator direct drive technology by Kollmorgen inside.
With Condrive, SMS Concast has developed a system which, due to the encapsulated direct drives from the KBM motor kit by Kollmorgen, is virtually maintenance-free. The innovation offers precision for controlling the oscillation of the casting die. In contrast to the other drive systems for oscillations (including hydraulic booster cylinders), Condrive combines a simple design with online settings and monitoring of the oscillation curves.