1. Hot rolling
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, which 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.
3. Hot rolled steel products are not well controlled for thickness and side width. We are familiar with thermal expansion and contraction. Since the hot rolling is started at the beginning even if the length and thickness are up to standard, there will be a certain negative difference after cooling. The wider the width of the negative difference, the thicker the thickness is. Therefore, for large steel, the side width, thickness, length, angle, and edge of the steel cannot be too precise.
It refers to the processing of steel plates or steel strips into various types of steel at room temperature by cold drawing, cold bending, cold drawing and other cold working.
The molding speed is fast, the output is high, and the coating is not damaged, and various cross-section forms can be made 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 in the forming process, there is still residual stress in the section, which will inevitably affect 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;
3. The cold-rolled steel has a small wall thickness and is not thickened at the corners where the plates are joined. 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.
There is not much difference between the two kinds of steel pipes in the finished product. They are generally manufactured according to the standard. Most of the mechanical properties are different. The processing technology is different, and the final precision is different. Generally speaking, the dimensional accuracy and surface finish of the cold drawing are generally small. Higher.
Hot rolling process:
Tube blank---heating---perforation---rolling tube---sizing---cold bed---straightening---cutting tube---inspection---packaging---shipping
Cold drawing process:
Tube blank---heating---perforation----hot rolling (two-wearing), etc.---heading--acid washing--phosphating--cold drawing--annealing (multiple times can be repeated to the beginning of the process )---straightening----cutting pipe---inspection---packaging---shipping