More than 40 years ago Japanese mills tried hot direct rolling from slabbing mill to hot strip mill.

After oil shock in 1973 oil price rose sharply. Japanese steel mills focused on reducing high priced energy cost and so fuel consumption reduction at reheating furnace became the primary problem to solve for hot strip mills. To reduce fuel at reheating furnace we started hot direct rolling from slabbing mill to hot strip mill skipping reheating furnaces. We tried lots of experiments to include hot direct rolling as one of a standard production way.

It was not easy to synchronize steel making operation and hot strip mill rolling campaign. I was the engineer in charge. One day just before the weekly down turn of the hot strip mill, hot direct rolling with 1580mm slabs were obliged to do, because some trouble, I do not remember the trouble at steel making now, caused the rolling chance much later than scheduled. So work rolls at the final rougher stand and finisher stands were worn out with lots of narrow materials already rolled and we might have flatness problem at finisher stands, I anticipated. I had to decide to stop the experiment or not.

I decided to continue the hot direct rolling experiment. I had been stationed at the slabbing mill pulpit and hurried to the finisher pulpit. When I reached near the final rougher, I found a cobble with head portion in earlier finisher stands. Extra deteriorated flatness was the cause of the cobble, finisher operators told me. We used nickel-grain roll at that time at final rougher work roll and the work roll’s wear was much bigger than I expected. Extra big sheet bar crown caused extra big center buckle at earlier stands which lead to a cobble.

This was the only one cobble that I made by my judgement as an engineer.