Development of a New Cooling Technology for Continuous Annealing

Gas jet cooling and water dip cooling have been used as the primary cooling methods in the continuous annealing line for cold-rolled steel sheets. However, these two methods are at the extremes of slow and rapid cooling, and their respective cooling rates can hardly be varied, whereas a new method of variable cooling capacity was needed far a continuous annealing line to be capable of processing wide range of strips. Therefore, the Accelerated Cooling (AcC) method was developed based on results of metallurgical research so as to cool any strip running at a line speed of 450m/min, at a cooling rate 100-200°C/sec from a temperature of about 700 to about 400°C (the end-point temperature) for overaging, and without deforming the strip. To develop this cooling method, a pilot line which can take commercial-width strip, though the line speed was low, 60m/min, was reconstructed. By developing a harmonica type gas-atomized water cooling nozzle (the AcC nozzle) and a way of arranging these nozzles into a cooling unit, and also by devising a control system utilizing the model-referenced adaptive control method and the Karman filtering method, the end-point temperature controlling of 400±5°C and the cooling rate controlling of 100-200± 15°C/sec were attained. This AcC system is operating very satisfactorily in the Hirohata and the Nagoya continuous annealing and processing lines (CAPL) of the Nippon Steel Corporation.