Cold rolling is processed and rolled on the basis of hot rolled coils. Generally speaking, it is a processing process of hot rolling---pickling---cold rolling. Hot rolled sheet has low hardness, easy processing and good ductility. Cold-rolled sheet has high hardness and is relatively difficult to process, but it is not easy to deform and has high strength. The strength of hot-rolled sheet is relatively low, the surface quality is poor (with oxidation/low finish), but the plasticity is good, generally medium and heavy plate, cold-rolled sheet: high strength, high hardness, high surface finish, generally thin plate, can be used as stamping Use a board.
The mechanical properties of hot-rolled steel sheets are far inferior to those of cold working and forging, but they have better toughness and ductility
Cold-rolled steel sheets have a certain degree of work hardening and low toughness, but they can achieve a good yield-to-strength ratio and are used to cold-form springs and other parts. At the same time, because the yield point is closer to the tensile strength, there is no danger during use. Predictability, accidents are prone to occur when the load exceeds the allowable load.
The difference between hot-rolled sheet and cold-rolled sheet can be attributed to the following points:
1. The cold plate is cold rolled and has no oxide skin on the surface, and the quality is good. The hot plate is hot-rolled and has oxide skin on the surface, and the thickness of the plate is different.
2. Rolling is divided into cold rolling and hot rolling, and the recrystallization temperature is the distinguishing point.
3. Hot-rolled sheet has poor toughness and surface flatness, and the price is lower, while cold-rolled sheet has good stretchability and toughness, but the price is more expensive. 4. The surface of the hot-rolled sheet without electroplating is dark brown, and the surface of the cold-rolled sheet without electroplating is gray. After electroplating, it can be distinguished from the smoothness of the surface. The smoothness of the cold-rolled sheet is higher than that of the hot-rolled sheet.
5. Cold rolling: Cold rolling is generally used to produce strip, and its rolling speed is high. Hot rolling: The temperature of hot rolling is similar to that of forging Product features of hot rolling and cold rolling:
Cold rolling: using hot-rolled steel coil as raw material, after pickling to remove oxide scale, cold rolling is carried out, and the finished product is hard-rolled coil. The plastic index decreases, so the stamping performance will deteriorate, and it can only be used for parts with simple deformation. Hard-rolled coils can be used as raw materials for hot-dip galvanizing plants, because hot-dip galvanizing units are equipped with annealing lines. The weight of the rolled hard coil is generally 20-40 tons, and the hot-rolled pickled coil is continuously rolled at room temperature. The inner diameter is 610mm. Product features: Because it has not been annealed, its hardness is very high (HRB is greater than 90), and its machinability is extremely poor. Only simple directional bending of less than 90 degrees (perpendicular to the coiling direction) can be performed. Cold rolling is generally annealed.
Advantages of hot rolling: 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 an isotropic body to a certain extent; the bubbles, cracks and porosity formed during casting can also be welded under the action of high temperature and pressure. Hot rolled disadvantages:
1. Hot-rolled steel products are difficult to control in terms of thickness and side width. We are familiar with thermal expansion and cold contraction. Because even if the length and thickness are up to standard when hot rolled at the beginning, there will still be a certain negative difference after cooling. The wider the side width of this negative difference, the thicker the thickness, the more obvious the performance. Therefore, for large steel, the width, thickness, length, angle, and edge of the steel cannot be too precise.
2. After hot rolling, the non-metallic inclusions (mainly sulfides and oxides, as well as silicates) inside the steel are pressed into thin sheets, and the phenomenon of delamination (interlayer) occurs. Delamination greatly deteriorates the tensile properties of the steel through the thickness, and there is the potential for interlaminar tearing as the weld shrinks. The local strain induced by the shrinkage of the weld often reaches several times the yield point strain, which is much larger than the strain induced by the load;
3. Residual stress due to uneven cooling. Residual stress is the stress of internal self-phase equilibrium without external force. Hot-rolled section steel of various sections has such residual stress. Generally, the larger the section size of the section steel, the greater the residual stress. Although the residual stress is self-balanced, it still has a certain influence on the performance of the steel member under the action of external force. For example, it may have adverse effects on deformation, stability, and fatigue resistance.
