In processing technology, hot rolling is hot working and cold rolling is cold working. Hot rolling is above recrystallization temperature, while cold rolling is below recrystallization temperature.
Application of cold rolling
Used in automobiles, refrigerators, washing machines and other household appliances, as well as industrial equipment and various building materials.
Application of hot rolling
1, structural steel
Production of common structural steel and welded structural steel, mainly used for the production of steel structures, bridges, ships and vehicles.
2. Weathering steel
Add special elements (phosphorus, copper, carbon, etc.). ), with good corrosion resistance and atmospheric corrosion resistance, is used to produce containers, special vehicles and building structures.
3. Steel for automobile structure
High strength steel plate has good tensile properties and weldability, and is used to produce automobile frames and wheels.
4. Hot rolled special steel
Carbon steel, alloy steel and tool steel for general mechanical structures are used to produce various mechanical parts after heat treatment.
5. Cold-rolled raw plate
Used to produce all kinds of cold-rolled products, including CR, GI, color-coated plates, etc.
6. Steel plates for steel pipes
With good processability and compressive strength, it is used to produce 500! The following high-pressure gas pressure vessels are filled with liquefied petroleum gas, acetylene gas and various gases.
7. Steel plates for high-pressure vessels
With good processability and compressive strength, it is used to produce 500! The following high-pressure gas pressure vessels are filled with liquefied petroleum gas, acetylene gas and various gases.
Extended data:
Concepts of cold rolling and hot rolling
Hot rolling 1
Continuous casting slab or initial rolling slab is used as raw material, which is heated by walking beam heating furnace, dephosphorized by high pressure water, and then enters the roughing mill, and the rough rolled material enters the finishing mill after cutting the head and tail, which is realized by computer. Control rolling, after rolling, use laminar cooling (cooling rate is controlled by computer), and use coiler to wind it into straight hair coil.
The head and tail of straight hair and curly hair are often tongue-shaped, fishtail-shaped, with poor accuracy in thickness, width and width, and often have defects such as wavy, folded edges and tower edges. Its coil weight, the inner diameter of steel coil is 760 mm (generally used by pipeline industry). ) The straight hair coil is processed by finishing lines of cutting head, tail, trimming, straightening and leveling for many times, and then cut into plates or rewinds, thus obtaining products such as hot-rolled steel plates, flat hot-rolled steel coils, longitudinal cutting belts and the like. If the hot-rolled finished coil is taken out after pickling and oiling, it becomes a hot-rolled pickled coil.
2. Cold rolling
Taking hot rolled steel coil as raw material, the oxide scale is removed by pickling, and then the finished product is hard coil. Because of the cold work hardening caused by continuous cold deformation, the strength and hardness of hard rolled coil increase, the toughness and plasticity index decrease, and the stamping performance deteriorates, so it can only be used for simple deformed parts. Because all hot dip galvanizing units are equipped with annealing units, hot rolled steel coils can be used as raw materials for hot dip galvanizing units.
Generally, the weight of hard rolled steel coil is 6~ 13.5 tons, and the inner diameter of steel coil is 610 mm. Generally, continuous annealing (CAPL unit) or bell furnace is used to anneal the coil for cold continuous rolling to eliminate cold working hardening and rolling stress and achieve the mechanical properties specified in the standard. The surface quality, appearance and dimensional accuracy of cold-rolled steel plate are better than those of hot-rolled steel plate.
Advantages and disadvantages of hot rolling:
Advantages: it can destroy the casting structure of steel ingot, refine the grain of steel, eliminate the defects of microstructure, make the structure of steel compact and improve the mechanical properties. This improvement is mainly reflected in the rolling direction, which makes the steel no longer isotropic to some extent; Bubbles, cracks and pores formed during pouring can also be welded under the action of high temperature and high pressure.
Disadvantages:
1. After hot rolling, nonmetallic inclusions (mainly sulfides and oxides, and silicates) in steel are pressed into thin sheets, resulting in delamination (interlayer). Delamination greatly deteriorates the tensile properties of steel along the thickness direction, and interlayer tearing may occur when the weld shrinks. The local strain caused by weld shrinkage often reaches several times the yield point strain, which is much greater than the strain caused by load.
2. Residual stress caused by uneven cooling. Residual stress is an internal self-balancing stress without external force, which exists in all kinds of hot rolled steel. Generally speaking, the larger the section size, the greater the residual stress. Although the residual stress is self-balanced, it still has some influence on the performance of steel members under external force. For example, it may adversely affect deformation, stability and fatigue resistance.
Advantages and disadvantages of cold rolling:
Advantages: high molding speed, high yield, no damage to the coating, various cross-sectional forms, and meeting the needs of use conditions; Cold rolling can cause great plastic deformation of steel, thus improving the yield point of steel.
Disadvantages:
1. Although there is no thermoplastic compression in the forming process, there is still residual stress on the cross section, which will inevitably affect the overall and local buckling characteristics of steel;
2. The style of cold-rolled steel is generally open section, which makes the free torsion stiffness of the section lower. Torsion is easy to occur when bending, and bending-torsion buckling is easy to occur when compressing, so the torsional performance is poor;
3. The wall thickness of cold-rolled steel is small, and it is not thickened at the corner where the plates are connected, so the ability to bear local concentrated load is weak.
References:
Baidu Encyclopedia-Cold Rolling and Hot Rolling