The Pit Charging Problem in Steel Production

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Abstract
In most integrated works, ingots are used as an intermediate form during the production of steel. These ingots, produced by pouring molten steel into molds, are too large to be used as input for the finishing mills which produce steel products. Hence, more manageable forms (blooms, billets and slabs) are rolled from the ingots by primary mills. The necessity of having uniformly hot steel, provided by a battery of soaking pits, limits the production capacity of any primary mill. At times there is no hot steel and the mill comes into “nothing hot” delays. It is desirable to reduce these delays to a minimum through proper management of the system. One way of doing so is to properly control the charging into the soaking pits of ingots from the queue which builds up there. The paper investigates the properties of this problem which can be considered in two parts. Firstly, when there are ingots of various temperatures in the queue, which should be charged when space becomes available in the pits? Secondly, if space becomes available when there are no hot ingots in the queue, should the empty pit be kept idle waiting for more to arrive or should it be charged with cold ingots which though readily available require considerably longer to prepare for rolling? There is the possibility of inserted idle time. A model is made of the ingot queue-soaking pit-rolling mill system. The properties of the optimal control policy for the ingot queue are investigated using Markov programming. Results are presented which illustrate its complex state dependent nature.

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