In general, hot rolling has better mechanical properties than cold rolling. The hot rolled steel has improved tensile strength, yield strength, ductility and toughness. In addition, it is important that heat treatment eliminates residual stresses. While cold rolling refers to the processing of steel sheets or strips into various types of steel by cold drawing, cold bending, cold drawing, etc. At normal temperature, which may result from uneven deformation of the steel and esidual stress
during processing, this residual stress has a great influence on stability. Both hot rolling and cold rolling are processes of forming steel or steel sheets, which have a great influence on the microstructure and properties of steel. The rolling of steel is mainly hot rolling, and the cold rolling is only used for producing small steel and thin steel.
The cast structure of the ingot can be destroyed, the grain of the steel can be refined, and the defects of the microstructure can be eliminated, so that the steel structure is compacted and the mechanical properties are improved. This improvement is mainly reflected in the rolling direction, so that the steel is no longer isotropic to a certain extent; bubbles, cracks and looseness formed during casting can also be welded under high temperature and pressure.
1. After hot rolling, non-metallic inclusions (mainly sulfides and oxides, as well as silicates) inside the steel are pressed into thin sheets, and delamination (sandwich) occurs. The delamination greatly deteriorates the tensile properties of the steel in the thickness direction, and it is possible to cause interlayer tearing when the weld is shrunk. The local strain induced by weld shrinkage often reaches several times the yield point strain and is much larger than the strain caused by the load.
2, Residual stress caused by uneven cooling. The residual stress is the internal self-phase equilibrium stress without external force. The hot-rolled steel of various sections has such residual stress. The larger the section size of the general steel, the larger the residual stress. Although the residual stress is self-phase-balanced, it still has some influence on the performance of steel members under external force. Such as deformation, stability, fatigue and other aspects may have adverse effects.
The molding speed is fast, the output is high, and the coating is not damaged, and various cross-section forms can be formed to meet the requirements of the use conditions;
Cold rolling can cause a large plastic deformation of the steel, thereby increasing the yield point of the steel.
1. Although there is no hot plastic compression during the forming process, residual stress still exists in the section, which inevitably affects the overall and local buckling characteristics of the steel.
2. The cold-rolled steel profile is generally an open section, so that the free torsional stiffness of the section is low. It is easy to twist when being bent, and it is prone to bending and torsion buckling when pressed, and the torsion resistance is poor. + Third, the wall thickness of the cold-rolled steel is small, and there is no thickening at the corners where the plates are joined, and the ability to withstand localized concentrated loads is weak.
The main differences between hot and cold rolling are:
1. Cold-rolled steel allows partial buckling of the section, so that the bearing capacity of the bar after buckling can be fully utilized; and the hot-rolled steel does not allow local buckling of the section.
2. The causes of residual stresses in hot-rolled and cold-rolled steels are different, so the distribution on the cross-section is also very different. The residual stress distribution on the section of the cold-formed thin-walled steel is curved, while the residual stress distribution on the section of the hot-striped steel or welded steel is a film type.