Q:What requirements do cold-rolled raw materials need to meet?
A:No iron oxide scale on the surface (removed by pickling after hot rolling);
Thickness tolerance ≤±0.05mm (hot rolling accuracy affects the accuracy of cold rolled finished products);
Uniform mechanical properties (avoiding strip breakage or uneven thickness during cold rolling).
Q:What is the preparation process before cold rolling?
A:1. Pretreatment:
Degreasing: If rolling oil or contaminants remain on the surface of the hot-rolled plate, it needs to be removed by alkaline cleaning (such as NaOH solution) or electrolytic degreasing to prevent contamination of the roller and the surface of the finished product.
Straightening: Straighten the wave shape and bend of the hot-rolled plate (using a roller straightener) to ensure that the plate shape is flat when unwinding.
2. Pickling (if the raw material is not pickled)
Purpose: Remove the iron oxide scale (Fe₃O₄, FeO, etc.) on the surface of the hot-rolled plate to avoid the oxide scale being pressed into the plate surface to form defects during rolling.
Process: Sulfuric acid pickling: Soak in 50~80℃ sulfuric acid solution;
Hydrochloric acid pickling: Normal temperature hydrochloric acid solution (concentration 10%~15%), the reaction is more efficient and suitable for continuous production lines;
Rinse and dry: After pickling, rinse the residual acid with clean water, and then dry it with hot air to prevent rust.
Q:What are the rolling methods and rolling mill types?
A:1. Single-stand cold rolling
Applicable scenarios: small-batch production or special specification products.
Equipment: four-roll reversible rolling mill (small working roll diameter, large support roll diameter, reducing rolling force and roll surface deflection).
Process characteristics: The steel plate is rolled back and forth in the rolling mill (forward rolling → reverse rolling → reverse rolling), gradually thinning;
The pass reduction rate (the thickness reduction ratio of a single rolling) is usually 10%~30%, and the total reduction rate can reach 60%~80%.
2. Continuous multi-stand cold rolling (cold tandem rolling)
Applicable scenarios: mass production of conventional thickness cold-rolled plates.
Equipment: 5~6-stand tandem rolling unit, equipped with AGC (automatic thickness control) and plate shape control (such as bending roll and roll shifting technology).
Process characteristics: The steel plate passes through multiple stands in sequence, each stand completes a certain amount of reduction, and the total reduction rate can reach 70%~90%;
The rolling speed is high, and the production efficiency is 5~10 times that of a single stand;
Q:What are the key process parameters during cold rolling?
A:Rolling force: The rolling force of a single stand can reach thousands of tons, and the rolling force of a single stand of cold rolling is usually 500~2000 tons; Excessive rolling force can easily lead to roller surface wear or steel plate edge cracking, which needs to be optimized through roller material and lubrication control.
Rolling speed: Single stand low-speed rolling is suitable for precision control; cold rolling high-speed rolling requires a high-speed tension control system.
Tension control: Apply tension between the stands of the continuous rolling mill and during coiling (front tension > rear tension), which includes reducing the rolling force and preventing the steel plate from slipping; inhibiting the deviation and wave shape of the steel plate and improving the plate shape;
Tension value range: Low carbon steel is about 10%~20% of the yield strength.
Q:What are the advantages and limitations of the cold rolling process?
A:Advantages
1. High precision: thickness tolerance can reach ±0.01mm, surface roughness Ra＜1μm, meeting the needs of precision manufacturing;
2. High strength: directly improve strength through work hardening, reducing subsequent heat treatment costs;
3. Diversified performance: flexibly adjust strength-plasticity matching through annealing process (such as deep drawing steel, high-strength steel).
Limitations
1. High energy consumption: cold rolling has high rolling force and energy consumption is about 2~3 times that of hot rolling;
2. Work hardening limitation: unannealed steel plates are difficult to deep draw and require additional annealing process;
3. Large equipment investment: the cost of cold rolling unit can reach hundreds of millions of yuan, which is suitable for large-scale production.