Science and technology
Cold Rolling Theory
Cold rolling refers to the rolling method of squeezing steel with the pressure of roll and changing the shape of steel at room temperature. Although the process also heats up the steel sheet, it is still called cold rolling. To be specific, hot rolled coil is used as raw material for cold rolling, and the product is hard rolled coil.
Generally cold rolled steel, such as galvanized steel, color steel plate must be annealed, so plastic and elongation rate is better, widely used in automobile, household appliances, hardware and other industries. The surface of cold rolled plate has a certain degree of smoothness, and the hands feel smooth, mainly the work of acid cleaning. Generally, the surface finish of hot-rolled plate cannot meet the requirements, so hot-rolled steel belt needs cold rolling, and the thickness of hot-rolled steel belt is generally 1.0mm, and cold rolled can reach 0.1mm. Hot rolling is above crystallization temperature, cold rolling is below crystallization temperature.
The change of cold rolling to steel shape belongs to continuous cold deformation. The cold hardening caused by this process increases the strength, hardness and toughness of the rolled coil.
For terminal use, cold rolling makes stamping performance worse. The product is suitable for simple deformed parts.
It can damage the casting structure of ingot, refine the grain of steel, and eliminate the defect of microstructure, so as to improve the mechanical properties of steel. The improvement is mainly reflected in the rolling direction, so that steel is not isotropic to some extent. Bubbles, cracks and looseness formed during pouring can also be welded together at high temperature and pressure.
1. After hot rolling, the non-metallic inclusions (mainly sulfide and oxides, as well as silicates) inside the steel are laminated and stratified. The lamination makes the performance of steel to be pulled along the direction of thickness greatly deteriorate, and it is possible to tear between layers when the weld seam shrinks. The local strain induced by weld shrinkage is often several times of the yield point strain, which is much larger than the strain caused by load.
2. Residual stress caused by uneven cooling. Residual stress is the stress of internal self-phase equilibrium without external force, which is found in hot-rolled section steel of various sections. Generally, the larger the section size of section steel, the greater the residual stress. Although the residual stress is self-balanced, it has some influence on the performance of steel members under external force. Such as deformation, stability, anti - fatigue and other aspects may have adverse effects.