Hot Rolled (HR) Coils

HOT ROLLED (HR) COIL

Hot rolling primarily involves deforming the slab/bloom at high temperature & roll pressure - The rolling temperatures vary from 900˚C to 1200°C, which is above the recrystallization temperature for most stainless steels.

The material runs through a series of roll with predetermined roll gaps, pressures, and speeds to achieve its finished dimensions while being at high temperatures.

High finishing temperature ensures the production of a uniform rolled product, increases the ability of the material to undergo thickness reductions up to 98%, and at the same time, prevents the possibility of unwanted strain hardening. After hot rolling, the long and thin sheets are spun into coils and left to cool, these are called Hot Rolled coils.

MAKING OF HRAP COILS

Hot rolling involves the following processes:

• Reheating:

It involves heating to above recrystallization temperature. The nonuniform initial cast grain structure of metal consists of large dendritic grains. They are usually brittle with weak grain boundaries and may contain defects such as shrinkage cavities, porosity caused by gases, and foreign materials such as metallic oxides. Reheating breaks the dendritic grain structures obtained during casting, giving rise to the formation of new uniform recrystallized grain structures.

• Roughing & Finishing:

The reduction is given up to approximately the required coil thickness from the initial slab/bloom thickness using roughing and finishing mill. The finishing mill can be either Steckel (reversible) or a Tandem (continuous) mill.

• Laminar cooling: In some grades, rapid cooling of hot rolled coils is done using water in order to bring down the temperature below 500°C

• Coiling: It is done in order to make the hot rolled sheet convenient for further processing and transportation.

FEATURES OF HR COILS

• Hot-rolled coils are often used in applications where dimensional tolerances are not very critical, such as railroads and construction materials.

• HR Black: The hot rolled coils develop a brownish-black oxide layer on the surface after processing, hence referred to as “HR Black coils.”

• HRAP: If a better surface finish is required, then oxide scaling can be removed by shot blasting (mechanically) and pickling (chemically) in order to obtain a No. 1 finish as per ASTM A480/A480M specifications. Hot rolled coils are annealed and pickled in continuous annealing-pickling lines having a scale breaker, shot blasting unit, electrolytic sulphuric acid, and mixed acid bath.

• HRAP coils have a cleaner surface and have improved mechanical properties for downstream processing.