Hot rolling and cold rolling are both processes for forming steel sections or steel plates. They have a great impact on the structure and properties of steel. Hot rolling is mainly used for steel rolling, and cold rolling is only used for Produces small steel sections and sheets.
1. Advantages of hot rolling: It can destroy the casting structure of the steel ingot, refine the grains of the steel, and eliminate defects in the microstructure, thereby making the steel structure dense and improving the mechanical properties. 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 the action of high temperature and pressure.
Disadvantages: 1. After hot rolling, the non-metallic inclusions (mainly sulfides, oxides, and silicates) inside the steel are pressed into thin sheets, resulting in delamination (sandwiching). . Delamination greatly deteriorates the tensile properties of the steel along the thickness direction and may cause interlaminar tearing as 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 load; 2. Residual stress caused by uneven cooling. Residual stress is the internal self-balanced stress in the absence of external force. Hot-rolled steel sections of various sections have such residual stress. Generally, the larger the cross-section size of the section steel, the greater the residual stress. Although residual stress is self-balanced, it still has a certain impact on the performance of steel components under the action of external forces. For example, it may have adverse effects on deformation, stability, fatigue resistance, etc.
2. Cold rolling refers to processing steel plates or steel strips into various types of steel through cold drawing, cold bending, cold drawing and other cold processing at normal temperature.
Advantages: fast forming speed, high output, and does not damage the coating. It can be made into a variety of cross-section forms to meet the needs of use conditions; cold rolling can cause large plastic deformation of steel. , thereby increasing the yield point of steel.
Disadvantages: 1. Although there is no hot 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 The open section results in a lower free torsional stiffness of the section. It is prone to torsion when bent and torsional buckling when pressed, and its torsional resistance is poor; 3. The wall thickness of cold-rolled steel is small, and there is no thickening at the corners where the plates are connected, so it can withstand localized The ability to concentrate loads is weak.
3. The main differences between hot rolling and cold rolling are:
1. Cold-rolled formed steel allows local buckling of the section, so that the bearing capacity of the rod after buckling can be fully utilized; Hot-rolled steel sections do 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 cold-formed thin-walled steel section is curved, while the residual stress distribution on the hot-rolled steel or welded steel section is film-type.
3. The free torsional stiffness of hot-rolled steel is higher than that of cold-rolled steel, so the torsional performance of hot-rolled steel is better than that of cold-rolled steel.
Extended information
Usually hot rolling lubricant is delivered to the surface of the roll in the form of a mixture of oil and water. Water is the carrier, and a small amount of oil is evenly dispersed in the water. The action process of oil-water mixture is the transformation process from oil-in-water phase to water-in-oil phase. After the mixed liquid reaches the roll surface, it quickly spreads on the roll surface in the form of oil-in-water. When it enters the deformation zone and contacts the high-temperature rolling stock, due to the effects of temperature and pressure, the water quickly evaporates and transforms into a water-in-oil phase. Part of the oil burns into combustion products mainly composed of ash; part of the oil evenly covers the contact arc surface between the roll and the rolled piece in the form of an oil film. Both can provide lubrication in the deformation zone within about 0.01s. role.
Cold rolling is to further thin the No. 1 steel plate to the target thickness at room temperature. Compared with hot-rolled steel plates, cold-rolled steel plates have a more precise thickness, smooth and beautiful surface, and have various superior mechanical properties, especially in terms of processing performance. Because cold-rolled raw coils are relatively brittle and hard and not suitable for processing, cold-rolled steel plates are usually required to be annealed, pickled and surface smooth before being handed over to customers. The maximum thickness of cold rolling is 0.1--8.0MM or less. For example, the thickness of cold-rolled steel plates in most factories is less than 4.5MM. The minimum thickness and width are determined according to the equipment capabilities and market demand of each factory.
Stainless steel cold-rolled steel plates are stainless steel plates produced by the cold rolling process. Those with a thickness of no more than 3mm are thin plates, and those with a thickness greater than 3mm are thick plates. It is used to make corrosion-resistant parts, pipelines, containers, medical equipment, ship equipment, etc. in petroleum and chemical industries. Its classification and grades are as follows:
1. Austenitic steel
In addition to the same as the hot rolled part (29 types), there are also: (1) 2Cr13Mn9Ni4 (2) 1Cr17Ni7 (3) 1Cr17Ni8
2. Austenitic-ferritic steel
In addition to the same as the hot-rolled part (2 types), there are also: (1) 1Cr18Ni11Si4AlTi (2) 1Cr21Ni5Ti
3. Ferritic steel
In addition to the same as the hot-rolled part (9 types), there are also: 00Cr17
4. Martensitic steel
In addition to the same as the hot-rolled part (8 types), there are also 1Cr17Ni2
5. Precipitation hardened steel: the same as the hot-rolled part
Reference materials: Baidu Encyclopedia - Cold-rolled stainless steel, Baidu Encyclopedia - Stainless steel hot-rolled steel plate