Cold Rolled Steel

Cold rolling refers to the rolling method of changing the shape of the steel by extruding the steel under the pressure of the roll at room temperature. Although the processing also heats up the steel sheet, it is still called cold rolling. Specifically, the hot-rolled steel coil for cold rolling is used as the raw material, the oxide scale is removed by pickling, and then pressure processing is performed, and the finished product is a hard-rolled coil.

cold rolled steel

Generally, cold-rolled steel such as galvanized steel and color steel plate must be annealed, so the plasticity and elongation are also good, and it is widely used in automobiles, home appliances, hardware, and other industries. The surface of the cold-rolled sheet has a certain degree of smoothness, and the hand feels smoother, mainly due to the pickling. The surface finish of the hot-rolled sheet generally does not meet the requirements, so the hot-rolled steel strip needs to be cold-rolled. The thinnest hot-rolled steel strip is generally 1.0mm, and the cold-rolled steel strip can reach 0.1mm. Hot rolling is rolling above the crystallization temperature point, and cold rolling is rolling below the crystallization temperature point.

The change of the shape of the steel by cold rolling belongs to continuous cold deformation, and the cold work hardening caused by this process increases the strength, hardness and toughness, and plastic index of the rolled hard coil.

For end use, cold rolling deteriorates stamping properties, and the product is suitable for simple deformation parts.

Advantage:

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 an isotropic body to a certain extent; the bubbles, cracks, and porosity formed during casting can also be welded under the action of high temperature and pressure.

Shortcoming:

1. After hot rolling, the non-metallic inclusions (mainly sulfides and oxides, as well as silicates) inside the steel are pressed into thin sheets, resulting in delamination. 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 the shrinkage of the weld often reaches several times the yield point strain, which is much larger than the strain caused 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, and fatigue resistance.

Summarize

The difference between cold rolling and hot rolling is mainly the temperature of the rolling process. “Cold” means normal temperature, and “hot” means high temperature.

From a metallographic point of view, the boundary between cold rolling and hot rolling should be distinguished by the recrystallization temperature. That is, rolling below the recrystallization temperature is cold rolling, and rolling above the recrystallization temperature is hot rolling. The recrystallization temperature of steel is 450 to 600°C.

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