Continuous annealing

The continuous annealing technique first appeared in 1936 (ARMCO in the USA ) for the production of cold-rolled sheet metal continuously galvanized by the Sendzimir process. This technique, which enabled sheet surface preparation and metallurgical annealing to be carried out simultaneously, developed rapidly from 1950 onwards.

Continuous annealing was also used at around the same time for thin, tinned cold-rolled strip for packaging, known as "tinplate", then for sheet metal for electrical applications, and for austenitic stainless steel sheet metal from 1960 onwards.

For these products, continuous annealing, in addition to the reliability advantages inherent in any continuous process due to the elimination of transients, was well suited to achieving the specified qualities.

The first continuous annealing line for uncoated cold-rolled sheet was commissioned in 1972 (Kimitsu Japan), but the process was not developed until 1982. This late development was due to both technical and metallurgical problems:

• development of continuous annealing lines for large-width thin strips;

• achieve qualities equivalent to those specified and achieved in coil annealing with regard to suitability for forming by stamping, and susceptibility to ageing after forming.

Coil annealing of thin sheet is a slow process:

• the thermal cycle lasts about 3 days;

• storage times between separate finishing operations last around 4 days.

Continuous annealing, developed over the past 40 years for annealing thin wafers for tinning lines and for annealing prior to hot-dip galvanizing (Sendzimir process), features a short thermal cycle of 1 to 3 minutes.

The extension of this technique to the annealing of thin sheet presented several advantages.

• Continuous annealing, skin pass, finishing and pre-shipment inspection operations can be carried out in around 10 minutes.

• Uncoiling the strip on a continuous annealing line cleans the strip surface before annealing and, as a result, reduces the surface carbon after annealing to below 3 mg per m ² . This low level of surface carbon is highly favorable for the quality of subsequent surface treatments.

• The elimination of separate operations eliminates the risk of damage caused by bumps or scratches during handling between annealing and finishing.

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