What are the differences between hot-rolled and cold-rolled steel?

Both hot rolling and cold rolling are major forming processes for steel plates or profiles, and both significantly affect the microstructure and properties of the steel. In actual production, hot rolling is the primary method of steel rolling, while cold rolling is mainly used for producing small-sized sections, thin plates, and other steel products with high dimensional accuracy requirements. There are significant differences between the two in terms of application scenarios, process characteristics, and performance, as detailed below.

 

 

I. Applications of Hot and Cold Rolling for Common Steel Products

Different steel profiles are rolled using different methods, mainly as follows:
* Wire rod: Diameter range 5.5-40 mm, in coil form, all hot-rolled; after cold drawing, it becomes cold-drawn material.

* Round steel: Except for dimensionally precise bright steel, most are hot-rolled; there are also forged materials with forging marks on the surface.

* Strip steel: Both hot-rolled and cold-rolled are produced, with cold-rolled materials typically being thinner.

Steel Plate: Cold-rolled plates are mainly thin and widely used in automobile manufacturing and other fields; hot-rolled plates are mostly medium and thick, and there are also specifications with similar thickness to cold-rolled plates, but the appearance is significantly different.

Angle Steel: All are produced by hot rolling.

Steel Pipe: Welded steel pipes are produced by both hot rolling and cold drawing. Seamless steel pipes can also be produced using hot rolling, cold rolling, or cold drawing processes, depending on the requirements.

Channel Steel and H-beams: All are hot-rolled materials.

Reinforcing Steel: The mainstream is hot-rolled material. Some hot-rolled reinforcing steel bars undergo cold drawing, cold working, etc. Cold-rolled ribbed reinforcing steel bars are a further processed product of hot-rolled round reinforcing steel bars.

 

II. Hot Rolling Process and Characteristics

(I) Process Definition

Steel ingots or billets are difficult to deform and process at room temperature, and usually need to be heated to 1100~1250℃ for rolling. This process is called hot rolling. The finishing temperature of hot rolling is generally 800-900℃, after which it is mostly cooled naturally in air. Therefore, the hot-rolled state is equivalent to normalizing the steel.

From a metallurgical perspective, hot rolling is rolling performed above the recrystallization temperature of steel (450-600℃). The general steel production process is: ironmaking-steelmaking-continuous casting (or ingot casting)-hot rolling (producing hot-rolled products)-cold rolling (producing cold-rolled products).

Most steel products are rolled using the hot rolling method. Hot-rolled steel, due to the high temperature, forms a layer of iron oxide scale on its surface, providing a certain degree of corrosion resistance and allowing for open-air storage.

(II) Advantages and Disadvantages

Advantages: It can disrupt the casting structure of steel ingots, refine the steel grains, eliminate microstructural defects, making the steel structure denser and improving its mechanical properties. This improvement is mainly reflected in the rolling direction; bubbles, cracks, and porosity formed during casting can be welded together under high temperature and pressure; it has a fast forming speed and high output, and can be processed into various cross-sectional shapes to meet different application requirements.

Disadvantages: The surface becomes rough due to iron oxide scale, resulting in significant dimensional fluctuations. Therefore, steels requiring high surface finish, dimensional accuracy, and mechanical properties must be further processed by cold rolling from hot-rolled semi-finished or finished products. After hot rolling, non-metallic inclusions (mainly sulfides, oxides, and silicates) inside the steel are compressed into thin sheets, leading to delamination. This significantly deteriorates the tensile strength of the steel along its thickness, potentially causing interlayer tearing during weld shrinkage. Uneven cooling generates residual stress; all types of hot-rolled steel exhibit this self-balancing residual stress, and the larger the cross-sectional size, the greater the residual stress, which may adversely affect the deformation, stability, and fatigue resistance of steel components.

 

III. Cold Rolling Process and Characteristics

(I) Process Definition

Cold rolling is a rolling method that uses the pressure of rolls to extrude steel at room temperature to change its shape. Although the deformation during processing generates heat that raises the temperature of the steel plate, it is still called cold rolling. Its production uses hot-rolled steel coils as raw materials, which are pickled to remove oxide scale before pressure processing, resulting in hard-rolled coils.

From a metallurgical perspective, cold rolling is rolling performed below the recrystallization temperature of steel. Typically, cold-rolled steel (such as galvanized steel sheet and color-coated steel sheet) requires annealing, resulting in better plasticity and elongation. Cold rolling allows for thinner thicknesses; hot-rolled steel strips are as thin as 1.0 mm, while cold-rolled strips can reach 0.1 mm.

(II) Advantages and Disadvantages

Advantages: Precise dimensions, uniform thickness, small tolerances, enabling the production of extremely thin products; good surface quality, controllable smoothness, and a smooth feel to the touch, mainly due to pickling treatment; good mechanical and technological properties (such as stamping properties), enabling high-speed, fully continuous rolling, and high production efficiency; the steel undergoes significant plastic deformation during cold rolling, which increases the yield point of the steel.

Disadvantages: Although no hot plastic compression occurs during the forming process, residual stress still exists within the cross-section, affecting the overall and local buckling characteristics of the steel; cold-rolled steel sections are mostly open sections with low free torsional stiffness, making them prone to torsion under bending and flexural-torsional buckling under compression, resulting in poor torsional resistance; cold-rolled steel has thin walls, and there is no thickening treatment at the joints of plates, weakening its ability to withstand localized concentrated loads; continuous cold deformation causes work hardening, leading to increased strength and hardness, decreased toughness and plasticity, and deteriorated stamping performance, making it only suitable for parts with simple deformation.

IV. Summary of Core Differences Between Hot-Rolled and Cold-Rolled Steel

The core difference between the two lies in the rolling temperature. "Cold" refers to room temperature (below the recrystallization temperature), while "hot" refers to high temperature (above the recrystallization temperature). The specific differences are mainly reflected in three aspects:

1. Appearance and Surface Quality: Since cold-rolled steel is obtained by cold-rolling hot-rolled steel, and cold rolling also involves some surface finishing, cold-rolled steel has better surface quality (such as surface roughness) than hot-rolled steel. Therefore, if there are high requirements for the quality of subsequent coatings, such as painting, cold-rolled steel is generally chosen. Hot-rolled steel is further divided into pickled and unpickled steel. Pickled steel has a normal metallic color due to the pickling process, but unpickled steel has a lower surface finish than cold-rolled steel. Unpickled steel usually has an oxide layer on the surface, appearing dull, or has a layer of iron oxide (Fe3O4). In layman's terms, it looks like it has been heated, and if the storage environment is poor, it will usually have some rust.

 

2. Performance: Generally, the mechanical properties of hot-rolled and cold-rolled steel sheets are considered identical in engineering applications. Although cold-rolled steel sheets undergo some work hardening during cold rolling (which may vary with stringent mechanical property requirements), they typically have slightly higher yield strength and surface hardness than hot-rolled steel sheets. The exact difference depends on the degree of annealing. Regardless of the annealing process, cold-rolled steel sheets generally have higher strength than hot-rolled steel sheets.

 

3. Formability: Since the properties of hot-rolled and cold-rolled steel sheets are essentially the same, the difference in formability is mainly due to differences in surface quality. Cold-rolled steel sheets generally have better surface quality, so for steel sheets of the same material, cold-rolled steel sheets usually have better formability than hot-rolled steel sheets.

 

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