First, hot rolling
Advantages: 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. 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 pouring can also be welded under high temperature and pressure.
Disadvantages: 1. After hot rolling, the non-metallic inclusions (mainly sulfides and oxides, as well as silicates) inside the steel are pressed into sheets, and the phenomenon of lamination (sandwich) appears. Delamination greatly worsens the tensile properties of steel along the thickness direction, and may cause interlayer tearing when the weld is contracted. The local strain induced by weld contraction often reaches several times of the yield point strain, which is much larger than that caused by load. 2. Residual stress caused by uneven cooling. Residual stress is the internal self-equilibrium stress in the absence of external force. Hot-rolled steel of various sections has such residual stress. The larger the section size of general steel, the greater the residual stress. Although the residual stress is self-equilibrium, it still has a certain effect on the performance of steel members under external forces. Such as deformation, stability, anti-fatigue and other aspects may have adverse effects. 3. Hot rolled steel products are not well controlled in terms of thickness and side width. We are familiar with thermal expansion and cold contraction, because at the beginning of hot rolling out even if the length and thickness are up to standard, there will be a certain negative difference after cooling, and the wider the negative difference edge width, the thicker the thickness, the more obvious the performance. Therefore, for large steel, the side width, thickness, length, Angle, and edge line of the steel can not be required to be too precise.
Two, cold rolling
It refers to the steel plate or steel strip processed into various types of steel at room temperature through cold drawing, cold bending, cold drawing and other cold processing.
Advantages: fast molding speed, high yield, and no damage to the coating, can be made into a variety of cross section forms to meet the needs of the conditions of use; Cold rolling can produce great plastic deformation of steel, thus increasing the yield point of steel.
Disadvantages: 1, although there is no hot plastic compression in the molding process, there is still residual stress in the section, which will inevitably affect the overall and local buckling characteristics of the steel; 2, cold rolled steel style is generally open section, so that the free torsional stiffness of the section is low. It is easy to appear torsion when bending, and easy to appear bending and torsional buckling when pressurized, and the torsional resistance is poor; 3. The wall thickness of cold-rolled steel is small, and it is not thickened at the corner of the connection of the plate parts, and the ability to withstand local concentrated load is weak.
Three, the main difference between hot rolling and cold rolling
1, the cold-rolled steel section allows local buckling, so as to make full use of the bearing capacity of the bar after buckling; Hot rolled steel does not allow local buckling of the section.
2. The causes of residual stress in 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-wall steel is curved, while the residual stress distribution on the section of hot-tied or welded steel is thin film.
There is not much difference between the two kinds of steel pipes in the finished product, and they are generally manufactured according to standards. There is no difference in mechanical properties, most of the existing processing technology is different, and the final accuracy is different, generally speaking, cold drawing dimensional accuracy and surface finish is higher.
Hot rolling process: tube billet - heating - perforating - rolling - sizing - cold bed - straightening - cutting - inspection - packaging - delivery
Cold drawing process: tube billet - heating - perforation - hot rolling (two wear), etc. - Head - pickling - phosphating - cold drawing - annealing (can repeat the head to the post process multiple times) - Straightening - tube cutting - inspection - packaging - delivery
As an important part of iron and steel products, steel pipe is divided into two categories: seamless steel pipe (round blank) and welded steel pipe (plate, strip blank) because of its manufacturing process and the shape of the tube blank used.
Because of its different manufacturing processes, seamless steel pipes are divided into hot-rolled (extruded) seamless steel pipes and cold-drawn (rolled) seamless steel pipes. Cold drawn (rolled) pipe is divided into two kinds: round pipe and shaped pipe.
Process overview
Hot rolling (extruded seamless steel pipe) : round tube billet - heating - perforation - three-roll skew rolling, continuous rolling or extrusion - stripping - sizing (or reducing) - cooling - billet - straightening - hydraulic test (or inspection) - marking - storage.
Cold-drawn (rolled) seamless steel pipe: round tube billet - heating - perforation - heading - annealing - pickling - oiling (copper plating) - multi-pass cold drawing (cold rolling) - billet - Heat treatment - straightening - hydraulic test (flaw detection) - marking - storage.
