Manufacturology

Improvement potential in steel plants

Why is it that so few steel plants practice hot charging, i.e. why is the material allowed to cool down after casting only to be reheated again in the next processing step?

Continuous casting is today the most frequently used method for production of steel workpieces. The continuous casting process was developed with high efficiency in mind. Why is it then that it often takes several days for a slab, produced to customer order, to cross the yard and enter the subsequent hot rolling processing step?

I see two main reasons:

1. Processing equipment is made for large batch production because focus is on manufacturing process efficiency instead of production system efficiency.
2. Jobs are batched in the rolling mill in order to create production schedules designed for optimal work roll (tool) utilisation instead of optimising flow.

In both cases the main problem is a focus on efficiency in individual manufacturing process steps instead of system efficiency.

A problem is that the interface between process steps is too often regarded as a matter of transportation and materials handling. Too little attention is therefore given to production and process improvement with the aim of improving the interface. However, in order to obtain best overall system (plant) performance, improvement of interfaces must be a prioritised issue for improvement initiatives in the neighbouring steps.

The time that slabs spend waiting depends on:

• The capability of the hot rolling mill to process slabs just-in-time as they arrive from casting.
• The capability of slab production to deliver only the slabs that are needed.

500 slabs in the "slab yard".

The above figure shows the interface between continuous casting (slab production) and hot strip rolling in a steel plant. As-cast slabs are stored in a slab yard before they are forwarded to the hot rolling mill where they are reheated (since they cooled down during storage). The amount of reheating needed depends on the time taken to transfer slabs from one processing step to the next. Longer transfer times implies more reheating and more wasted heat (energy).

Assume that the average transfer time is 5 days (120 hours). With a mean casting rate of 83 tonne per hour, there will be 83t/h × 120h ≈10000t material in in-process inventory. With an average slab weight of 20tonnes, this means 500 slabs.

Needless to say, the cost for tied capital, slow flow and energy losses due to cooling are enormous.

I think that it is time for researchers and industry to start developing equipment that permits efficient production of small batches of steel. I also question if maximisation of tool utilisation is the recipe for best plant economy. Rolling mills should break free from traditional scheduling practices and learn the benefits of just-in-time production and continuous flow.