The production process for cold-rolled strip steel primarily involves controlling billet preparation, pickling, cold rolling, annealing, and finishing.
Billet preparation requires that chemical composition, dimensional tolerances for width and thickness (specifically "three-point difference" and "intra-strip variation"), and camber all meet specified requirements. Furthermore, the surface must be smooth and free from defects such as cracks, folds, laminations, pores, and non-metallic inclusions.
Prior to pickling, the steel strip must be straightened and butt-welded to facilitate a continuous pickling operation. The primary objective of pickling is to remove iron oxide scale. During the pickling process, strict control must be maintained over the concentration and temperature of the acid solution, as well as the content of ferrous salts within the solution.
To control thickness and strip shape, adjustments are made to the roll reduction, rolling speed, strip tension, and roll profile. Thickness is primarily controlled via Automatic Gauge Control (AGC), while strip shape is predominantly controlled by adjusting the roll profile-specifically through roll crowning and crown compensation measures-using technologies such as HC (Hydraulic Crown) or CVC (Continuous Variable Crown) rolls.
Annealing is categorized into intermediate annealing and final annealing. Intermediate annealing serves to relieve work hardening, whereas final annealing is performed to achieve the desired microstructure and mechanical properties. Annealing furnaces typically fall into two categories: continuous annealing furnaces and bell-type annealing furnaces. In bell-type furnaces, the annealing process requires controlling the ratio of protective gases within the furnace, as well as the heating and cooling durations; in continuous furnaces, the process requires controlling temperature, speed, time, and atmosphere in strict accordance with a specific annealing curve. Controlling the tension of the steel strip within the furnace helps ensure proper strip shape, while controlling the crown of the furnace rolls prevents the strip from drifting laterally.
Finishing operations encompass leveling, shearing, oiling, and packaging. Leveling serves to improve strip shape, clean the surface, and achieve specific mechanical properties. During the leveling process, the elongation of the steel strip must be carefully controlled. Shearing operations focus primarily on controlling dimensional accuracy and surface quality. Oiling must be applied uniformly, and packaging must comply with prescribed standards to facilitate proper storage, transportation, and delivery.
