There are some necessary processes such as hot rolling, cold rolling, cold drawing, cold drawing in the steel processing. Hot rolling, as opposed to cold rolling, is rolled above the recrystallization temperature. Hot rolling can destroy the casting structure of ingot, refine the grain of the steel, eliminate the microstructure defects and improve the mechanical properties. The defects such as bubbles, cracks and porosity formed during pouring can also be improved under high temperature and pressure. Cold rolling is the process by which round steel is rolled on a cold rolling machine into a bar with a regular cross-section shape, which can improve its strength and adhesion to concrete. In the process of cold rolling, the steel bar deforms both vertically and laterally, so its plasticity and internal structure uniformity can be well maintained. Cold drawing means that the steel bar is pulled to any point in the strengthening stage of its σ-ε curve, and then the load is slowly unloaded. When the steel bar is reloaded, its yield limit will be increased, while its plastic deformation ability will be reduced. The general yield point of the steel bar can be increased by 20%~50% after cold drawing. We are here today to focus on cold and hot rolled steel pipes.
Hot-rolled steel pipe
Hot rolled steel pipe is the pipe made by the hot-rolling process. The fine crystal particles inside the hot rolled steel melt and become uniform because of the high temperature, thus improving the density and strength of the forging, further changing the mechanical characteristics of the steel, can withstand greater pressure, the increase in density makes the steel contact with airless, corrosion slows down. Hot rolling processes are always:: perforated tube -billet heating — rolling tube sizing – cooling – straightening-cutting test. But hot rolling has some disadvantages:
- After hot-rolled steel, non-metallic inclusions (mainly sulfides and oxides, as well as silicate) are compressed and thin, resulting in the lamination (interlayer) phenomenon. Delamination greatly worsens the tensile properties of the steel along with the thickness and may cause interlaminar tearing as the weld shrinks. The local strain induced by weld shrinkage is often several times the yield point strain, which is much larger than that caused by the load.
- Residual stress caused by uneven cooling. Residual stress is the internal self-equilibrium stress in the absence of external force, and all kinds of hot-rolled sections have this kind of residual stress. Generally, the larger the section size of the section, the greater the residual stress, which may have adverse effects on deformation, stability, fatigue resistance and other aspects.
- Hot rolled steel plate is not easy to control the thickness and width. Hot rolled steel plate cooling will appear a certain negative difference, the wider the width, the thicker the steel plate performance is more obvious. So for large steel side width, thickness, length, Angle, are not set too precise specifications.
Cold working methods of steel pipes include cold rolling, cold drawing and spinning, which are mainly used to produce small diameter, precision, thin wall and high strength pipes. Cold rolled steel pipe has a smooth surface, accurate size, good performance, many cross-section shapes and a high utilization rate. It is widely used in the fields of military industry, machinery, mining, chemical industry, electric power, agricultural machinery and so on. The main advantages of cold-rolled steel pipe are large section reduction rate, especially strong wall reduction ability. For carbon steel, the reduction of the section can reach 80% ~ 83% in a single rolling, and for alloy steel, it can reach 72% ~ 75%.
But there are some inherent drawbacks to cold rolling:
- Although there is no hot plastic compression in the forming process, there is still residual stress in the section, which affects the overall and local buckling characteristics of the steel;
- The wall thickness of cold-rolled steel is small, and there is no thickening at the corner of the plate connection, and the ability to bear the localized concentrated load is weak.
The appearance and mechanical properties of hot-rolled and cold-rolled steel tubes are almost the same. The only difference lies in the processing technology and precision. Generally speaking, the section of cold-rolled steel allows local buckling, so that the bearing capacity of the bar after buckling can be fully utilized. Local buckling of sections is not allowed for hot-rolled steel pipes. Cold drawing has the highest dimensional accuracy and surface finish. Some people often confuse cold drawing and cold rolling. In fact, cold drawing is a kind of processing technology of material, which is the forced drawing of the smooth round steel bar through the hard alloy wire drawing die hole. After one or more cold drawings, the yield point of cold-drawn low carbon steel wire can be increased by 40%~60%, but it loses the plasticity and toughness of soft steel and has the characteristics of hard steel. Cold drew steel pipe with hot rolled steel coil as raw material, after pickling to remove the oxide scale after cold tandem rolling, the finished product is rolled hard coil, because of continuous cold deformation caused by cold hardening hard rolled coil strength, hardness, toughness and plastic index decreased, so the stamping performance will deteriorate, can only be used for simple deformation of parts.
But they have a few things in common. For example, they all work at room temperature, below the temperature at which metals recrystallize. It’s all done by plastic deformation of the metal through dislocation motion. In the process of plastic deformation, the resistance of dislocation movement mainly comes from the dislocation itself.
However, in cold working, the strength and hardness of the metal are increased due to the strengthening of dislocation interaction, the increase of dislocation density and the increase of deformation resistance during mechanical plastic deformation. The difference is this:
- The use of compressive stress in cold rolling is relatively high in precision. Generally, cold rolling is some thin-walled tubes, can process the steel sheet, strip, foil.
- The use of cold pull tensile stress, precision is slightly poor, can cold draw some of the thin-walled tube and thick-walled tube is not less than 3.5 mm. Can be processed products are cold drawn round steel, steel bar, square, hexagonal steel and so on.
- Cold drawing refers to the method that the material is pulled out through a mold hole by applying a drawing force at one end of the material, and the diameter of the mold is smaller than the diameter of the material. Cold drawing processing makes the material in addition to tensile deformation and extrusion deformation, generally in a special cold drawing machine.