How to distinguish between hot-rolled stainless steel pipes and cold-rolled stainless steel pipes

Hot-rolled stainless steel pipes and cold-rolled stainless steel pipes are both processes of forming steel sections or stainless steel plates. They have a significant impact on the microstructure and properties of stainless steel. The rolling of stainless steel includes hot-rolled and cold-rolled stainless steel plates. 

Hot-rolled stainless steel sheet 

The advantages of hot-rolled stainless steel plates are that they can destroy the casting structure of the steel ingot, refine the grains of the stainless steel material, and eliminate the defects in the microstructure, thereby making the metallographic structure of the steel more dense and improving its mechanical properties. This improvement is usually mainly manifested along the rolling direction, thus making the stainless steel plates no longer be isotropic to a certain extent. At the same time, it can also weld together the bubbles, cracks and porosity formed during the previous casting process under high temperature and pressure. 

However, hot-rolled stainless steel plates also have drawbacks. After undergoing hot rolling, the non-metallic inclusions (mostly sulfides, oxides, and silicates) within the stainless steel sheet will be compressed into thin sheets, resulting in a layering phenomenon. Layering significantly deteriorates the tensile properties of the steel along the thickness direction, and there is a possibility of inter-laminar tearing during weld shrinkage. The local strain induced by weld shrinkage often reaches several times the yield point strain, which is much stronger than the strain caused by loads. Secondly, uneven cooling during hot rolling can generate residual stress. Residual stress is the stress that is self-balanced internally without external force. Various sections of hot-rolled steel sections have this residual stress. Generally, the larger the cross-sectional size of the steel section, the greater the residual stress. Although residual stress is self-balancing, it still has an impact on the performance of stainless steel components under external forces. For example, it is unfavorable for the deformation, stability, and fatigue resistance of stainless steel. 

Cold-rolled stainless steel sheet 

The advantages of cold-rolled stainless steel plates are that they can be formed quickly and can be made into various cross-sectional shapes to meet the requirements of usage conditions; cold rolling can also cause significant plastic deformation in the steel, thereby increasing the yield point of the stainless steel plates. 

However, its drawbacks do exist. Although the forming process did not involve hot plastic compression, residual stress still exists within the cross-section, which will inevitably affect the overall and local buckling characteristics of the stainless steel sheet; Secondly, the cold-rolled steel sections are generally of open cross-sections, resulting in a relatively low free torsional stiffness of the cross-section. During bending, torsion is prone to occur, and during compression, bending-torsion buckling is prone to happen, with poor torsional performance; Moreover, the wall thickness of cold-rolled formed steel is relatively small, and there is no thickening at the corner where the plates connect, which leads to the weak ability of the stainless steel sheet to bear local concentrated loads. 

The main differences between hot-rolled stainless steel pipes and cold-rolled stainless steel pipes 

Cold-rolled formed stainless steel pipes are allowed to have local buckling in their cross-sections, thereby enabling the full utilization of the post-buckling bearing capacity of the members. However, hot-rolled steel type steel pipes are not permitted to have local buckling in their cross-sections. 

2. The causes of residual stress in hot-rolled steel sections and cold-rolled steel sections are different, resulting in significant differences in the distribution on the cross-sections. The distribution of residual stress on the cross-sections of cold-formed thin-walled steel sections is of the bending type, while the distribution of residual stress on the cross-sections of hot-rolled steel sections or welded steel sections is of the film type. 

3. The free torsional stiffness of hot-rolled steel sections is higher than that of cold-rolled steel sections. Therefore, the torsional resistance of hot-rolled steel pipes is superior to that of cold-rolled steel.