What is hot rolling? What is cold rolling?

1. Hot rolling

Use continuous casting or initial rolling slab as raw material, heat it in a step-type heating furnace, remove phosphorus with high-pressure water, and then enter the rough rolling mill. The rough rolling material is cut and then enters the finishing mill for computer-controlled rolling. After the final rolling, it is cooled by laminar flow and coiled by the coiler to become a straight coil.

The head and tail of the straight coil are often tongue-shaped and fish-tail-shaped, with poor thickness and width accuracy, and the edges often have defects such as wave shape, folded edge, and tower shape. The coil weight is heavy and the inner diameter of the steel coil is 760mm. After the straight coil is cut, cut, trimmed, and processed by the finishing line for multiple straightening and flattening, it is cut or re-rolled to become: a hot-rolled steel plate, flat hot-rolled steel coil, longitudinally cut strip, and other products.

If the hot-rolled finishing coil is pickled to remove the oxide scale and oiled, it becomes a hot-rolled pickled coil.

2. Cold rolling

Using a hot-rolled steel coil as raw material, cold rolling is carried out after pickling to remove the oxide scale. The finished product is the hard rolled coil. Due to the cold work, and hardening caused by continuous cold deformation, the strength and hardness of the hard rolled coil increase, the toughness and plasticity index decrease, and the stamping performance deteriorates. It can only be used for parts with simple deformation.

The hard rolled coil can be used as raw material for hot-dip galvanizing plants because hot-dip galvanizing units are equipped with annealing lines. The weight of the hard rolled coil is generally 6~13.5 tons, and the inner diameter of the steel coil is 610mm.

Generally, cold-rolled plates and coils should be annealed by continuous annealing (CAPL unit) or bell furnace to eliminate cold work hardening and rolling stress and meet the mechanical properties specified in the standard. The surface quality, appearance, and dimensional accuracy of cold-rolled steel plates are better than those of hot-rolled plates.

3. Main differences

The main differences between cold rolling and hot rolling are processing technology, application range, mechanical properties, surface quality, and price differences.

(1) Processing technology. Hot rolling is carried out at a high temperature, while cold rolling is carried out at room temperature. Hot rolling is rolling performed above the crystallization temperature, while cold rolling is rolling performed below the crystallization temperature.

(2) Application scope. Hot-rolled steel is mainly used in steel structures or mechanical parts, including bridge construction, while cold-rolled steel is more commonly used in the automotive industry or small household appliances, washing machines, refrigerators, etc., including building materials.

(3) Mechanical properties. The mechanical properties of cold rolling are usually better than those of hot rolling, because the work hardening effect or cold work hardening produced during the cold rolling process results in higher surface hardness and strength of cold-rolled sheets, but lower toughness, while the mechanical properties of hot-rolled sheets are far inferior to those of cold-rolled sheets, but they have better toughness and ductility.

(4) Surface quality. The surface structure quality of cold-rolled steel is better than that of hot-rolled steel. Cold-rolled products are harder and less ductile, while the surface of hot-rolled products is rougher and has textures. 

Specification thickness. Cold-rolled coils are usually thinner than hot-rolled coils. The thickness range of cold-rolled coils is 0.3~3.5 mm, while that of hot-rolled coils is 1.2~25.4 mm.

(5) Price. Generally speaking, the price of cold rolling is slightly more expensive than hot rolling. This is because cold rolling requires more sophisticated processing equipment and more complex process technology, and cold rolling can achieve better surface treatment effects, so the quality of cold-rolled products is generally higher and the price is correspondingly higher. In addition, cold-rolled steel requires more stringent processing technology and higher processing difficulty during the production process and has higher requirements for production equipment, rollers, and other equipment, which will also lead to an increase in production costs. 

Product Details:

Our factory: