1.What are the drawbacks caused by excessive speed?
Inadequate pickling (not thoroughly cleaned):
Symptom: Unreacted iron oxide scale (black scale) remains on the strip surface, distributed in strips or blocks.
Consequences: This is the most feared problem in the cold rolling process. The residual iron oxide scale is extremely hard and will be pressed into the strip matrix during subsequent cold rolling, forming an embedded scale defect. This can result in black spots on the finished product surface or, in severe cases, direct strip breakage.
Forced increase in acid temperature: To compensate for speed, the acid temperature often needs to be increased. This exacerbates acid mist volatilization, wastes costs, and may lead to localized over-pickling.
2.What are the drawbacks caused by excessively slow speed?
Over-pickling:
Phenomenon: The acid not only dissolves the iron oxide scale but also begins to react with the steel matrix (iron).
Consequences: A pickled pitting forms on the surface, resulting in microscopic unevenness and a sharp increase in roughness. More seriously, over-pickling can lead to hydrogen embrittlement-hydrogen atoms generated from the reaction between acid and iron penetrate the steel matrix lattice, potentially causing the strip to become brittle and crack during subsequent rolling or stamping.
Bladder Hot Deformation:
Prolonged immersion in the acid bath and excessive heating time can easily cause thermal warping, i.e., uneven heating in the transverse direction of the strip leads to wavy edges or wrinkles in the middle, affecting coiling quality.
3.What are the impacts of velocity fluctuations on mass?
More fatal than being "fast" or "slow" is inconsistent speed:
Uneven pickling at the beginning and end: Modern pickling lines use "turbulent pickling," and speed variations disrupt the flow of acid on the strip surface. This results in different pickling times at the beginning and end of a coil, creating a segmented color difference with black at the beginning and end and white in the middle.
Uncontrolled tension in the process section: Speed fluctuations cause tension fluctuations in the inlet and outlet loopers. Unstable tension causes the strip to vibrate in the pickling tank, scratching the squeeze rolls and idler rolls. These scratches are longitudinal and cannot be eliminated by cold rolling.
4.What are the principles for controlling ideal speed?
Steel grade determines pickling speed:
High-silicon steel/high-carbon steel: The iron oxide scale is dense, requiring a slow pickling speed.
Low-carbon mild steel: The oxide scale is porous, allowing for a fast pickling speed.
Temperature and concentration compensation: Higher acid temperatures and concentrations allow for faster pickling speeds; however, when the acid solution is aged (high iron content), the speed must be reduced.
Scale breaking machine effect: If the elongation setting of the inlet straightening machine (scale breaking machine) is large, the oxide scale is already broken up, and the pickling speed can be significantly increased.
5.What is the essence of pickling speed?
The essence of pickling speed is the balance between "chemical reaction time" and "mechanical operating efficiency." For cold-rolled coils, under-pickling results in absolute scrap (they cannot be rolled further), while over-pickling poses a potential risk (performance degradation). Therefore, on-site operations typically follow the principle of "better slow than fast, better under-pickling than over-pickling" (within a controllable range).