Differences between Cold Rolling and Hot Rolling

Cold Rolling vs. Hot Rolling:

1) Differences in technology

1. Cold-rolled section steel allows local buckling of the section, so that the bearing capacity of the member after buckling can be fully utilized; while hot-rolled section steel does not allow local buckling of the section.
2. The reasons for the residual stress of hot-rolled steel and cold-rolled steel are different, so the distribution on the cross-section is also very different. The residual stress distribution on the section of cold-formed thin-walled steel is curved, while the residual stress distribution on the cross-section of hot-rolled or welded steel is thin-film.

3. The free torsional stiffness of hot-rolled steel is higher than that of cold-rolled steel, so the torsion resistance of hot-rolled steel is better than that of cold-rolled steel.

2) Differences in advantages and disadvantages

Cold rolling refers to the processing of steel plates or strips into various types of steel through cold working such as cold drawing, cold bending, and cold drawing at room temperature.

Advantages of cold rolling: 

Fast forming speed, high output, and no damage to the coating, it can be made into a variety of cross-sectional forms to meet the needs of the conditions of use; cold rolling can cause great plastic deformation of the steel, thereby improving the The yield point of steel.

Disadvantages of cold rolling:
1. Although there is no hot plastic compression in the forming process, there is still residual stress in the section, which will inevitably affect the overall and local buckling characteristics of the steel;
2. The style of cold-rolled steel is generally an open section, which makes the free torsional stiffness of the section low. It is prone to torsion when under bending, prone to bending-torsional buckling under compression, and has poor torsional performance;
3. The wall thickness of cold-rolled forming steel is small, and it is not thickened at the corners where the plates are connected, and the ability to withstand local concentrated loads is weak.

Hot-rolled seamless pipes are compared with cold-rolled seamless pipes. Cold-rolled seamless pipes are rolled below the recrystallization temperature, while hot-rolled seamless pipes are rolled above the recrystallization temperature.

Advantages of hot rolling:
It can destroy the casting structure of the ingot, refine the grain of the steel, and eliminate the defects of the microstructure, so that the steel structure is dense and the mechanical properties are improved. This improvement is mainly reflected in the rolling direction, so that the steel is no longer isotropic to a certain extent; the bubbles, cracks and porosity formed during casting can also be welded under the action of high temperature and pressure.

Disadvantages of hot rolling:
1. After hot rolling, the non-metallic inclusions (mainly sulfides and oxides, as well as silicates) inside the steel are pressed into thin sheets, and the phenomenon of delamination (interlayer) occurs. Delamination greatly deteriorates the tensile properties of the steel through the thickness, and there is the potential for interlaminar tearing as the weld shrinks. The local strain induced by weld shrinkage often reaches several times the yield point strain, which is much larger than the strain induced by the load;

2. Residual stress caused by uneven cooling. Residual stress is the stress of internal self-phase equilibrium without external force. Hot-rolled section steel of various sections has such residual stress. Generally, the larger the section size of the section steel, the greater the residual stress. Although the residual stress is self-balanced, it still has a certain influence on the performance of the steel member under the action of external force. For example, it may have adverse effects on deformation, stability, fatigue resistance, etc.

3. For hot-rolled steel products, it is difficult to control the thickness and side width. We are familiar with thermal expansion and cold contraction. Even if the length and thickness are up to standard at the beginning of hot rolling, there will still be a certain negative difference after cooling. The wider the side width of this negative difference, the thicker the thickness, the more obvious the performance. Therefore, for large steel, the width, thickness, length, angle, and edge of the steel cannot be too precise.

Hot rolling vs. cold rolling: How to choose?

(1) When hot rolling is preferred
Low cost required: Hot rolling is a simple process with low energy consumption and a price 10%-30% lower than cold rolling.
Thick materials: Hot rolling can produce thick plates (3-200mm), while cold rolling is usually ≤5mm.
Structural parts/non-precision applications: such as building steel beams, ship decks, pipelines, etc.
High toughness required: such as bridges, pressure vessels and other impact-resistant scenes.

(2) When cold rolling is preferred
High surface quality requirements: such as car shells, home appliance panels, and precision instruments.
Thin plate requirements (<3mm): Cold rolling can produce ultra-thin plates (thinnest 0.1mm).
High strength + formability: such as automotive structural parts (deep drawing is required).
Strict dimensional tolerances: such as electronic component brackets and precision mechanical parts.

Cost and delivery considerations:
Hot rolling: short production cycle (direct rolling), suitable for large-volume orders.
Cold rolling: requires multiple processes (pickling, rolling, annealing), delivery time is extended by 20%-50%, and the cost is higher.

Conclusion:
Choose hot rolling: low cost, thick plates, impact-resistant structural parts.
Choose cold rolling: high precision, thin plates, high surface requirements or deep processing.
Mixed use: Some industries use hot-rolled billets first, and then cold-rolled for finishing (such as automotive steel plates).

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