What is work hardening of cold-rolled coils?
A: After cold rolling, the grains of the strip are flattened and elongated, the crystal lattice becomes distorted and the grains break down, leading to a decrease in the metal's plasticity and an increase in its strength and hardness. This increases the steel's resistance to deformation, making subsequent cold rolling more difficult. Most strips require recrystallization annealing to eliminate this phenomenon.
What are the hazards and causes of insufficient pickling in cold-rolled coil production?
A: The hazards range from a dark or mottled surface after rolling to severe cases where oxide scale is pressed in, forming black spots, causing product waviness or warping, and increasing the scrap rate. Causes include uneven oxide scale thickness, large strip waviness and camber leading to localized missed pickling, and incomplete oxide scale removal before pickling.
What are the effects of over-pickling on cold-rolled coils?
A: If the metal remains in the acid solution for too long, the surface will gradually become rough and pitted, damaging the surface smoothness of the cold-rolled coil, affecting the accuracy and appearance quality of subsequent processing, and may also reduce the material's localized corrosion resistance.
What is strip breakage due to excessive speed in cold-rolled coil production? How can it be avoided?
A: When rolling to the fourth pass, the starting coil diameter is small. If the speed increases too quickly, exceeding the coiler's maximum speed, strip breakage will occur. The solution is to keep the speed below the coiler's maximum speed and increase it only after the coil diameter has reached a certain value.
What causes rust in cold-rolled coil production?
A: It is mainly caused by residual acid solution on the strip surface after pickling, or by incomplete drying after cleaning. This allows the strip surface to oxidize upon contact with air and moisture, leading to the re-appearance of rust.