Hot rolled:
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.However, after hot rolling, the non-metallic inclusions (mainly sulfides and oxides, but also silicates) inside the steel are pressed into thin sheets and delamination occurs. The local strain induced by weld shrinkage often reaches several times the yield point strain, which is much larger than the strain induced by the load.And the residual stress caused by uneven cooling. 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, fatigue resistance, etc.

Cold rolled
It refers to the processing of steel plates or steel strips into various types of steel through cold working such as cold drawing, cold bending, and cold drawing at room temperature.The advantage is that the forming speed is fast, the output is high, and the coating is not damaged, and the cross-section forms are various to meet the needs of the use conditions;Cold rolling can cause a large plastic deformation of the steel, thereby increasing the yield point of the steel.However, although there is no hot plastic compression during the forming process, there is still residual stress in the section, which will inevitably affect the overall and local buckling characteristics of the steel.It is prone to torsion during bending and bending-torsional buckling during compression. The wall thickness of cold-rolled forming steel is small, and there is no thickening at the corners where the plates connect, so the ability to withstand local concentrated loads is weak.

The difference

1. Cold-rolled section steel allows local buckling of the section, so that the bearing capacity of the member after buckling can be fully utilized; while hot-rolled section steel does not allow local buckling of the section.

2. The reasons for the residual stress of hot-rolled steel and cold-rolled steel are different, so the distribution on the cross-section is also very different. The residual stress distribution on the section of cold-formed thin-walled steel is curved, while the residual stress distribution on the cross-section of hot-rolled or welded steel is thin-film.

3. The free torsional stiffness of hot-rolled steel is higher than that of cold-rolled steel, so the torsional performance of hot-rolled steel is better than that of cold-rolled seamless pipe