What is cold rolling process

Cold rolling is an important metal processing method, widely used in the forming and processing of various metal materials such as steel, aluminum and copper. Compared with hot rolling, cold rolling is a secondary processing of metals at room temperature. Through rolling, not only can the strength and hardness of metal materials be significantly enhanced, but also their dimensional accuracy and surface quality can be effectively improved. The following introduces to you the steps, characteristics, applications and other aspects of the cold rolling process, helping you understand what the cold rolling process is.

The principle of cold rolling process

Cold rolling is the process of repeatedly compressing metal sheets and strips that have undergone hot rolling and pickling at room temperature through rolls, gradually reducing their thickness to the required size. Due to processing under low-temperature conditions, the grain structure inside the metal changes, thereby enhancing its strength and hardness while maintaining good plasticity and toughness.

Cold rolling process flow

1. Raw material preparation: The raw materials for cold rolling are generally hot-rolled steel. Before the start of cold rolling, the hot-rolled steel needs to undergo acid washing treatment to remove the oxide scale on its surface.
2. Rolling: The pickled metal sheets are sent into the cold rolling mill and undergo multiple roller rolling processes to gradually compress the metal sheets to the required thickness and width. During the rolling process, metal sheets are subjected to strong compression by the roller conveyor, which refines their grain structure and enhances the material's strength and hardness.
3. Annealing: During the rolling process, metal sheets are exposed to high temperatures and pressures, causing changes in their internal structure. The material's strength increases, but its plasticity decreases. To restore plasticity and improve performance, annealing treatment needs to be carried out in a protective atmosphere.
4. Flattening: After annealing, the metal sheet is slightly rolled to eliminate its yield elongation, improve its surface quality and dimensional accuracy.
5. Surface treatment and finishing: After the rolling process is completed, the plates need to undergo end cutting, oiling, anti-rust treatment, packaging, etc.

The characteristics of cold rolling process

High precision: The thickness tolerance of cold-rolled steel can be controlled within ±0.01mm or even smaller.
Excellent surface quality: The surface of cold-rolled products is smooth and bright, making them suitable for coating or deep processing.
Improving mechanical properties: Each rolling process can increase the hardness of the steel and give it a relatively high strength. However, the plasticity of the steel decreases after cold rolling. Therefore, multiple annealing processes are required to balance the strength and plasticity of the steel.

Industrial application cases

Automobile manufacturing
Cold-rolled duplex steel (DP600) is used for A-columns and B-columns, reducing weight by 20% compared to traditional steel and enhancing the anti-intrusion ability by 40%.
The aluminum engine block achieves a wall thickness of 0.8mm through cold rolling, with a lightweight rate of 35%.

Electronic industry
0.03mm thick oxygen-free copper foil is used for chip carrier tape, with a conductivity of ≥100%IACS.
Stainless steel cold-rolled sheet (304) is used for mobile phone casings, with a surface roughness of Ra0.4μm, meeting the requirements of PVD coating.

New energy field
Zinc-aluminum-magnesium steel (ZAM) is used for photovoltaic brackets, with a corrosion resistance 10 times that of ordinary galvanized sheets and a service life of over 50 years.
Copper-aluminum composite plates (rolled composite) are used as heat sinks for lithium batteries, with a thermal conductivity of up to 380W/m·K.

What should be noted when cold rolling different metals?

Steel: Cold rolling of steel requires strict control over work hardening (single-pass reduction rate ≤20%, total deformation ≤70%). Pickling must thoroughly remove iron oxide scales (hydrochloric acid concentration 15%-20%), and rolling oil containing sulfur and phosphorus extreme pressure agents (friction coefficient ≤0.08) should be used.
Aluminum materials: For cold rolling of aluminum materials, the rolling temperature (20-60℃) and the pass reduction rate (≤50%) need to be controlled. Emulsion lubrication (oil-water ratio 1:15-20) should be adopted and hard chrome-plated rolls (Ra≤0.2μm) should be used to prevent roll sticking.
Copper materials: Cold rolling of copper materials requires precise control of annealing temperature (300-400℃ for oxygen-free copper and 450-550℃ for brass) and oxygen content (< 5ppm). The final pass reduction rate should be ≤10% to ensure that the surface roughness Ra is ≤0.05μm.

Read more：What is hot rolling process or What is the meaning of cold rolling and hot rolling in steel pipes?