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Energy and Heavy Industry

Hot Strip Rolling Mills

Video thumbnail: Inside a hot rolling mill 6:30

Inside a hot rolling mill. Filmed and published by Högskolan Dalarna. Plays from YouTube, nothing here is re-hosted.

What it does

A hot strip mill takes a slab of steel the size of a shipping container, heated until it glows, and squeezes it into a coil of strip a few millimetres thick and hundreds of meters long. It is the process that makes most of the flat steel in the world.

How it works

The slab is soaked in a reheat furnace until it is uniformly around 1,200 degrees Celsius. Descaling sprays blast the oxide off with high pressure water. A roughing mill reduces the slab through several passes into a long transfer bar. That bar then enters the finishing train, a line of six or seven four-high stands set only five or six meters apart and close coupled so the strip is gripped in every stand at once. Because volume is conserved and the strip gets thinner at each stand, it must move faster at each stand, so all the stands are speed-locked to each other by the control system and any error shows up instantly as a loop or a tear. Out of the last stand the strip crosses a run-out table under laminar cooling sprays that set the metallurgy, then a coiler winds it up.

The numbers

The strip leaves the last finishing stand at roughly 600 to 1,200 meters per minute, which is 10 to 20 meters a second. Coils are wound on a 750 mm inside diameter to an outside diameter of up to 2,400 mm, at up to around 220 kg per centimetre of width, with coil weights that can exceed 30 tonnes. A mis-coil, called a cobble, is one of the more dramatic industrial failures anyone can film.

Who uses one

Integrated steel mills and mini mills supplying automotive, appliance, construction, pipe and tube, and general fabrication markets.

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