1) Hot rolled seamless steel pipe
Advantages: It can destroy the casting structure of the steel 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 isotropic to a certain extent; the bubbles, cracks and looseness formed during casting can also be welded under high temperature and pressure.
1. After hot rolling, the non-metallic inclusions (mainly sulfides and oxides, as well as silicates) inside the steel are pressed into thin sheets, and delamination (interlayer) occurs. Delamination greatly deteriorates the tensile properties of the steel in the thickness direction, and it is possible that interlayer tearing may occur when the weld shrinks. The local strain induced by weld shrinkage 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 internal self-balanced stress without external force. Hot-rolled steel sections of various cross-sections have such residual stresses. Generally, the larger the section size of the steel section, the larger the residual stress. Although the residual stress is self-balanced, it still has a certain influence on the performance of steel components under external force. For example, it may have adverse effects on deformation, stability, and fatigue resistance.
3. Hot-rolled steel products are not easy to control in terms of thickness and edge width. We are familiar with thermal expansion and contraction. Because at the beginning, even if the length and thickness are up to the standard, there will be a certain negative difference after the final cooling. The wider the negative difference, the thicker the thickness, the more obvious the performance. Therefore, for large steel, the side width, thickness, length, angle, and sideline of the steel cannot be too precise.
2) Cold rolled seamless steel pipe
Advantages: fast forming speed, high output, and without damaging the coating, it can be made into a variety of cross-sectional forms to meet the needs of use conditions; cold rolling can cause large plastic deformation of steel, thereby improving the yield of steel point.
1.Although there is no thermal plastic compression during the forming process, there are still residual stresses in the section, which will inevitably affect the overall and local buckling characteristics of the steel;
2. The style of cold-rolled steel is generally an open section, which makes the free torsional stiffness of the section lower. It is prone to torsion when being bent, and it is prone to bending and torsion buckling when being compressed, and the torsion resistance is poor;
3. The wall thickness of cold-rolled steel is small, and there is no thickening at the corners where the plates join, and the ability to withstand local concentrated loads is weak.
3) The main differences between hot rolling and cold rolling are:
1. The cold-rolled formed steel allows local buckling of the section, so that the bearing capacity of the bar after buckling can be fully utilized; while the hot-rolled steel does not allow local buckling of the section.
2. Hot-rolled steel and cold-rolled steel have different causes of residual stress, so the distribution on the section is also very different. The residual stress distribution on the cross-section of cold-formed thin-walled steel is curved, while the residual stress distribution on the cross-section of hot: rolled steel 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 torsion resistance of hot-rolled steel is better than that of cold-rolled steel.