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.

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