What steps does the complete cold-rolled coil production process include?
The core process is: hot-rolled coil raw material → pickling → cold rolling → annealing → skin pass → finishing (slitting and coiling) → inspection → finished product.
Pickling: Removes the oxide scale (Fe₂O₃, Fe₃O₄) on the surface of the hot-rolled coil to prevent scratches on the rolls and steel plate during rolling.
Cold rolling: Continuously rolls the hot-rolled coil through multiple cold rolling mills, reducing the thickness from 3-12mm to 0.1-3mm (adjustable based on demand).
Skin pass: Lightly rolls the steel plate through a skin pass mill (reduction ratio of 1-3%) to improve surface flatness and adjust yield strength.
Finishing: Slitting into cut-to-length steel plates (e.g., 1220mm x 2440mm) or recoiling into coils based on customer requirements.
How does "rolling force" during cold rolling affect product quality? Rolling force is the pressure exerted by the cold rolling mill on the steel plate. Its magnitude directly affects:
Thickness accuracy: Insufficient rolling force will result in excessive plate thickness, while excessive force may deform the rollers, also affecting accuracy;
Surface quality: Uneven rolling force can easily cause surface defects such as "horizontal striations" and "wavy patterns";
Mechanical properties: Excessive rolling force intensifies the work hardening effect, reducing the steel's plasticity and requiring a more aggressive annealing process to restore it.
What acids are commonly used in the pickling process? How can pickling quality be controlled?
Commonly used acids are hydrochloric acid (HCl) or sulfuric acid (H₂SO₄). Hydrochloric acid offers high pickling efficiency, excellent surface quality, and is therefore more widely used. Key factors in controlling pickling quality:
Acid concentration: Hydrochloric acid concentration is typically controlled between 18-22%. A too low concentration results in incomplete pickling, while a too high concentration can corrode the steel substrate.
Pickling temperature: Room temperature to 80°C (the higher the temperature, the faster the pickling process, but avoid excessive acid mist).
Pickling time: Adjust according to scale thickness (usually 3-5 minutes). Too short a time will leave scale, while too long can lead to "over-acidification" (pitting) on the steel surface.
Why is "protective atmosphere annealing" necessary in cold-rolled coil production?
Ordinary air annealing causes surface oxidation (scale formation) on the steel plate, affecting subsequent processing and appearance. Therefore, a protective atmosphere (such as nitrogen or a hydrogen + nitrogen mixture) is required:
It isolates oxygen and prevents oxidation of the steel plate.
Hydrogen reduces tiny oxidized impurities on the steel plate surface, improving surface finish.
It ensures uniform microstructure after annealing and prevents scale from affecting mechanical properties.
What is the function of a skin-pass mill? How does it differ from a cold rolling mill?
The core function of a skin-pass mill is to "finish the surface," not "reduce thickness." The differences between a skin-pass mill and a cold rolling mill are as follows:
Function: The cold rolling mill focuses on "reducing thickness" (reduction ratio 10-80%), while the skin-pass mill focuses on "improving the surface" (reduction ratio 1-3%).
Purpose: The cold rolling mill determines the final thickness of the steel plate, while the skin-pass mill eliminates the "yield plateau" after cold rolling (avoiding "slip lines" during stamping) and corrects surface undulations.
Equipment Structure: The skin-pass mill's rollers are more precise (producing a smoother surface finish) and offer lower rolling forces.