Coilbox Upgrade Strategies Involving Passive Transfer
Presented at the Hatch 2008 Coilbox Symposium, October 12, 2008, Beijing, China.
Upgrading to a mandrel-less forced transfer system can be completed in two stages. The first stage deals with immediate mandrel shortfalls by either partially or fully limiting its use. The second stage will involve the complete removal of the mandrel equipment and the addition of articulated cradle rolls and heat shields to further improve performance.
A passive transfer Coilbox system can be implemented as a first stage upgrade towards a mandrel-less forced transfer system. This paper presents an approach for quantifying the throughput capacities of said passive transfer systems. The push-passive system is also introduced for dealing with higher paced rolling schedules.
The onset of passive transfer is calculated as a function of strip thickness, yield stress, and cradle roll geometry; wherein yield stress is dependent on temperature, strain, strain rate, and equivalent carbon content according to Shida . Results for an example mill indicate the onset is sooner with thicker bars than with thinner bars. Therefore, the maximum throughput capacity of a passive transfer system is achieved when rolling thicker and wider bars. By comparison with the capacities of other operating equipment; such as the reheat furnace(s), roughing mill, and finishing mill; the product size limitations of a passive transfer system are identified. For products larger than these sizes, the capacity of a passive transfer system will not be limiting; for smaller products, passive transfer will be limiting and a push-passive system should be considered.