Hot rolling and cold rolling are two vital forming processes for steel plates and profiles, which significantly alter steel’s internal microstructure and mechanical properties. What are their essential differences?
Steel production mainly relies on hot rolling. Cold rolling is only adopted to manufacture precise-sized products including small-sized section steel and thin steel sheets.
Common Steel Products: Hot Rolled VS Cold Rolled
- alambrón: Diameter 5.5~40mm, coiled form, todo laminado en caliente; cold drawn wire belongs to cold processed material after secondary drawing.
- Round bar: Most are hot rolled except precision bright finish round steel; forged round bars carry visible forging marks on surfaces.
- Tira de acero: Both hot rolled and cold rolled types available; cold rolled strip is generally thinner.
- Placa de acero: Cold rolled sheets are thin, widely used for automobile panels; medio & heavy plates are mostly hot rolled with distinct surface appearances.
- Ángulo de acero: 100% hot rolled.
- Steel pipe: Welded pipes can be hot rolled or cold drawn.
- Channel steel & viga H: Hot rolled production only.
- Reinforcing bar: Pure hot rolled product.
Hot Rolling Process Introduction
Steel ingots and steel blanks have poor plasticity at ambient temperature and are hard to deform. Hot rolling refers to rolling steel after heating raw materials to 1100℃~1250℃.
The finishing temperature of hot rolling ranges from 800℃ to 900℃, followed by natural air cooling. The finished hot rolled steel is equivalent to receiving normalizing heat treatment.
Most steel products are manufactured via hot rolling. A dense iron oxide scale forms on the surface under high temperature, providing basic corrosion resistance so hot rolled steel can be stored outdoors.
Nevertheless, the oxide scale leads to rough surface and large dimensional tolerance. For steel requiring smooth surface, tight dimensional accuracy and superior mechanical performance, hot rolled semi-finished coils are used as raw materials for subsequent cold rolling.
Ventajas del rodillo caliente
- Fast forming speed, high production output, no damage to surface coating; various cross-section profiles can be customized to match diverse service conditions.
- Severe plastic deformation occurs during hot rolling, effectively raising the yield strength of steel.
Disadvantages of Hot Rolling
- Residual internal stress remains inside the cross-section without hot plastic compression, negatively impacting global and local buckling resistance of steel components.
- Most hot rolled sections are open profiles with low free torsional rigidity, prone to torsion under bending load and flexural-torsional buckling under compression, resulting in weak torsion resistance.
- Hot rolled steel features thin wall thickness with no thickening at plate joint corners, leading to poor bearing capacity under local concentrated loads.
Cold Rolling Process Introduction
Cold rolling shapes steel by continuous roller extrusion at room temperature. Although friction heat raises plate temperature during processing, it is still classified as cold forming. Específicamente, cold rolling takes hot rolled steel coils as feedstock, removes oxide scale via pickling, and conducts pressure forming to produce hard rolled coils.
Cold rolled steel such as galvanized steel and color coated steel must go through annealing treatment, delivering excellent plasticity and elongation. These materials are widely used in automotive, home appliance and hardware industries. Cold rolled plates feature smooth, bright surfaces thanks to pickling procedures. Hot rolled strip cannot meet high surface finish standards and thus requires cold rolling processing. The minimum thickness of hot rolled strip is about 1.0mm, while cold rolled strip can reach as thin as 0.1mm.
Metallurgically, hot rolling is performed above steel recrystallization temperature, while cold rolling operates below recrystallization temperature.
Continuous cold deformation during cold rolling causes work hardening, increasing tensile strength and hardness of hard rolled coils but reducing ductility and elongation.
For end-use manufacturing, work hardening deteriorates stamping performance, making cold rolled steel suitable only for parts with simple forming deformation.
Advantages of Cold Rolling
- Breaks down the coarse casting microstructure of steel ingots, refines grain size and eliminates microstructural defects, achieving compact steel texture and optimized mechanical properties. The property improvement is more prominent along rolling direction, making steel anisotropic.
- Bubbles, microcracks and porosity generated during casting can be welded under high temperature and rolling force.
Disadvantages of Cold Rolling
- Non-metallic inclusions (sulfides, oxides, silicates) inside steel are flattened into thin sheets after hot rolling, triggering delamination. Delamination drastically weakens through-thickness tensile performance and may cause interlayer tearing during weld shrinkage. Local strain induced by welding shrinkage can be several times higher than yield strain from external loads.
- Uneven cooling generates residual internal stress, which maintains self-equilibrium inside steel without external force. All hot rolled sections contain residual stress, and larger section dimensions correspond to higher residual stress levels. Though self-balanced, residual stress impairs component deformation stability and fatigue resistance under external loads.
Core Distinction Standard: Recrystallization Temperature
The fundamental difference between cold rolling and hot rolling lies in rolling temperature: cold rolling proceeds at room temperature, while hot rolling uses high-temperature processing.
From metallurgical theory, the dividing line is steel recrystallization temperature: rolling below 450℃~600℃ recrystallization temperature is cold rolling; rolling above this threshold is hot rolling.
Three Major Practical Differences Between Hot Rolled & Acero laminado en frío
1. Surface Appearance & Finalizar
Cold rolled plates are produced from hot rolled coils with extra surface finishing, delivering far superior surface roughness and smoothness. Cold rolled sheets are preferred if subsequent painting or coating quality is critical.
Hot rolled steel includes pickled and non-pickled grades: pickled hot rolled steel presents standard metallic luster yet inferior smoothness vs cold rolled steel; non-pickled hot rolled steel is covered with dark oxide or magnetite layers, easily developing rust under poor storage conditions.
2. Mechanical Performance
In general engineering applications, hot rolled and cold rolled steel are considered equivalent in mechanical properties. Cold rolled steel carries work hardening effect; cold rolled sheets have slightly higher yield strength and surface hardness depending on annealing degree. Even after full annealing, cold rolled steel maintains higher strength than hot rolled counterparts. Special mechanical performance requirements demand separate material selection.
3. Forming Performance
Given similar base mechanical parameters, forming quality mainly depends on surface finish. Cold rolled steel with smoother surfaces achieves better stamping and forming effects for identical steel grades.