Why do hot rolled strips need further cold rolling process?
Why do hot rolled strips need further cold rolling process?
This post discusses the role of the cold rolling operation.
In the melt shop, desirable material is formed: a chemical composition is created. Casting then follows in order to transform the liquid into a solid state.
Under old methods, the form created after casting was the ingot. Square sections were used, and the weight could reach several tons. The ingots were then rolled at high temperature to give birth to slabs. The thickness of the slab ranged from 100 to 150 mm. The width depended on the final requirement, and the length depended on the weight of the slab.
A second hot rolling was then necessary to transform the slab into strip. For this step, the slab was heated again at high temperature and passed through several mill stands, sometimes 12 stands. The exit product was wound strip.
The minimum resulting strip thickness was 1.6 mm for soft material (such as mild steel) and 2.5 mm for hard materials such as stainless steel. The maximum thickness for strip was about 8 mm.
By the way, hot rolling cannot produce thin and wide strip. Because of mechanical limitations, thin gauges can only be obtained in narrow strip.
Today, ingots have all but disappeared (except in very special cases), thanks to continuous casting. The solid state is first obtained in the slab.
In the future, hot rolling will be completely eliminated. Strip will be produced by continuous casting. This goal has already, in all practicality, been reached for mild steels.
Hot-rolled strip is the raw material for flat products. However, thin gauges, high surface quality, high accuracy of mechanical properties and thickness, and good shape require that the operations belonging to the “cold rolling complex” be carried out. These operations are:
• annealing to modify the structure,
• pickling to clean the surface, • slitting to obtain the required width, and
• cold rolling to reduce the thickness.
Process rout of typical cold rolling complex:
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