What Is the Difference Between Hot Rolled Steel and Cold Rolled Steel?

Hot rolling and cold rolling are processes used to shape steel plates or profiles, significantly impacting the structure and properties of the steel.

Hot rolling is primarily used for steel production, while cold rolling is typically reserved for smaller profiles and thin sheets.

Common situations for hot and cold rolling of steel include:

• Wire: Usually hot rolled, then cold drawn for precision.

• Round steel: Mostly hot rolled, with some forged.

• Strip steel: Both hot and cold rolled, with cold rolled generally thinner.

• Steel plate: Cold rolled plates are usually thinner, like those for automobiles, while hot rolled plates are thicker with a visibly different appearance.

• Angle steel: Typically hot rolled.

• Steel pipes: Welded pipes can be hot rolled or cold drawn.

• Channel steel and H-shaped steel: Hot rolled.

• Reinforcing bars: Hot rolled.

Hot Rolling: Steel ingots or billets are difficult to deform at room temperature, so they are heated to 1100-1250°C for rolling, a process known as hot rolling. The final temperature is usually 800-900°C, followed by cooling in the air, similar to annealing.

Most steel is hot rolled, forming a layer of iron oxide on the surface, providing corrosion resistance but resulting in rough surfaces and dimensional fluctuations. For smoother, more precise, and mechanically robust steel, hot rolled semi-finished or finished products are cold rolled.

Advantages: 

Fast forming, high yield, and versatile shapes can be achieved without damaging coatings. Cold rolling increases steel's plastic deformation, improving its yield point.

Disadvantages: 

Residual stresses persist despite no hot plastic compression during forming, impacting overall and local buckling characteristics. Cold-formed steel profiles tend to have low torsional rigidity, making them prone to twisting under bending and less resistant to torsion.

Cold Rolling: 

Cold rolling involves shaping steel at room temperature using pressure from rolling mills. Although some heating occurs during processing, it's still called cold rolling. Typically, hot rolled steel coils are acid-pickled and then cold-rolled to produce hardened coils.

Cold rolled steel, such as galvanized and color-coated steel, usually requires annealing, resulting in better plasticity and elongation, making it widely used in automotive, household appliance, and hardware industries. Cold rolled plates have smoother surfaces due to acid pickling. Hot rolled plates often lack the required surface finish, requiring cold rolling. Hot rolled steel strip thickness typically starts at 1.0mm, while cold rolling can achieve 0.1mm.

Advantages: 

Cold rolling breaks down cast steel's grain structure, refining grains, and eliminating defects, resulting in denser steel with improved mechanical properties. This improvement, mainly along the rolling direction, leads to steel becoming somewhat anisotropic.

Disadvantages: 

Residual stresses from uneven cooling affect performance under external loads, potentially impacting deformation, stability, and fatigue resistance.

In summary, the main difference between hot rolling and cold rolling lies in the temperature during rolling. "Hot" refers to above-recrystallization temperature, while "cold" refers to room temperature. Recrystallization temperatures typically range from 450 to 600°C for steel. Hot and cold rolling processes have distinct characteristics, influencing surface quality, performance, and formability, with cold rolling often preferred for better surface quality and forming performance.