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Production Methods > Hot Strip Mill

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Hot rolled bands and sheets are primarily low carbon steels that are produced on a Hot Strip Mill from slabs that have been thoroughly heated to a rolling temperature of approximately 2300°F.  Slab heating at this temperature accomplishes several desirable end results, which include, surface scaling for minor surface imperfection removal, softening of the steel so it can be hot rolled, and (where applicable) dissolving carbides or nitrides that are to be precipitated at a later stage of the rolling process.

After departing the reheat furnace, the slab enters a high pressure water spray descaling unit then is subjected to a series of 4 or 5 "roughing stands" that successively reduce the slab thickness from approximately 9-inches to 1-inch. As the slab becomes thinner and longer by the horizontal rolls in the roughing train, the slab edges are also squeezed by vertical rolls in order to control width. Upon exiting the last stand of the roughing train, the slab ends are sheared so a rectangular shape or "transfer bar" enters the 6 or 7 finishing stands that further reduce the thickness of the transfer bar to the final hot rolled band or sheet gauge desired. The last hot working operation is usually conducted above the upper critical temperature of 1450°F which permits the rolled steel to pass through a phase transformation after all the hot work is finished. This produces a uniformly fine, equiaxed grain throughout all of the steel. If part of the hot rolling is conducted on steel that has already partially transformed to ferrite, the deformed ferrite grains will coarsen during the self-anneal that occurs during the cooling of the coiled steel. This condition could limit the suitability of the steel for some severe drawing applications. Attributes imparted to the strip by the hot rolled strip mill include surface quality, thickness, width and flatness control as well as strip profile.

After leaving the last finishing stand, of the hot strip mill, the rolled strip is delivered across a runout table that is equipped with top and bottom cooling sprays.  By controlling the amount, location and flow of cooling water to which the strip is exposed, the mechanical properties and shape of the hot rolled strip can be affected.

From the runout tables, the hot rolled strip enters mandrel type coilers.  Air operated pushers remove the coils from the mandrels then place them onto a coil tilter. The coil tilter positions the coils, with their diameters upright, onto chain conveyor that transports the coils to the coil storage area. The hot coil chain conveyor is equipped with a heavy-duty scale that accurately measures and records the weight of each coil

1. Reheat Furnaces
Modern furnaces, with the aid of computer controls, uniformly heat continuous cast slabs to the desired temperature of approximately 2300^oF.  Uniform heating provides for improved rollability, gauge control and excellent metallurgical properties.

2. Scale Breaker and Descaling Sprays
The oxide formed during reheating is removed to provide a clean surface condition for rolling in the roughing stands.

3. Roughing Stands
Reduce the slab thickness from 8"-10" to approximately 1" for final processing in the Finishing Mills. In addition, these mills control the entry width into the Finishing Mills and are equipped with high pressure water descaling to assure an excellent surface.

4. Crop Shear
Shears the product prior to entering the Finishing Mill to provide square head and tail ends for rolling.

5. Descaling Unit
Located immediately ahead of the Finishing Mill. This high pressure descaling unit removes any surface oxide to provide a clean surface for rolling in the Finishing Mill.

6. Finishing Mill
Reduces the incoming 1" thick product to the final ordered thickness through successive reductions in a series of closely coupled mill stands.

7. Runout Table Cooling
Modern laminar water flow cools the strip from its Finishing Mill exit temperature to a specified coiling temperature. The cooling rate and final coiling temperature are controlled in order to obtain the desired uniform metallurgical properties.

8. Coilers
Wrap the finished strip on mandrels to provide a tight coil with uniformly straight side walls for either shipment or further processing.

This is representative of the typical surface appearance and edge condition of
Hot Rolled Sheet Non-Temper Rolled product.

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