Home >News> Difference Between Cold Rolling and Hot Rolling of Steel
Difference Between Cold Rolling and Hot Rolling of Steel
The difference between cold rolling and hot rolling of steel
Advantages of hot rolling: can destroy the foundry structure of steel ingots
Disadvantages of hot rolling: after hot rolling, inclusions
Advantages of cold rolling: fast forming speed and high output
Disadvantages of cold rolling: no hot plastic compression
Basic concepts of hot rolled and cold rolled steel coils
1. Hot rolling: Continuous casting slab or slab slab is used as raw material, heated by step heating furnace, high-pressure water dephosphorization and then enters the rough rolling mill. The rough rolling material passes through the cutting head and tail and then enters the finishing mill to implement The computer controlled the rolling, and after the final rolling, it was subjected to laminar cooling (computer controlled cooling rate) and the coiler was taken up to become a straight hair roll. The head and tail of the straight hair roll are often in the shape of a tongue and a fishtail. The thickness and width are poorly accurate, and the side has frequent defects such as a wave shape, a folded edge, and a tower shape. The weight of the coil is heavier and the inner diameter of the coil is 760 mm. (Generally, the tube industry likes to use it.) After the straight hair is wound through the cutting, cutting, trimming and multi-pass straightening, leveling and other finishing lines, then cutting or rewinding, it becomes: hot rolling Steel plates, flat hot rolled steel coils, slitting belts and other products. If the hot-rolled finishing roll is pickled to remove the scale and oiled, the hot-rolled pickled sheet roll is formed.
2. Cold rolling: hot-rolled steel coil is used as raw material, and the scale is removed by acid pickling, followed by cold continuous rolling. The finished product is rolled hard roll. The cold work hardening caused by continuous cold deformation increases the strength and hardness of the rolled hard roll. The toughness and plasticity index is lowered, so the punching performance will deteriorate and it can only be used for parts that are simply deformed. Rolled hard rolls can be used as raw materials for hot-dip galvanizing plants because the hot-dip galvanizing units are provided with an annealing line. The hard roll weight is generally 6~13.5 tons, and the inner diameter of the coil is 610mm. Generally, cold-rolled steel coils should be subjected to continuous annealing (CAPL unit) or hood furnace annealing to eliminate cold work hardening and rolling stress, and achieve the mechanical properties specified in the corresponding standards. The surface quality, appearance and dimensional accuracy of the cold-rolled steel sheet are superior to those of the hot-rolled sheet, and the thickness of the product is thinned to about 0.18 mm.
Advantages and disadvantages of hot rolling:
Advantages: It can damage the foundry structure of steel ingots, refine the grain of steel, and eliminate the defects of microstructure, so that the steel structure is compact 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 casting can also be welded under high temperature and pressure.
Disadvantages: 1. After hot rolling, non-metallic inclusions (mainly sulfides and oxides, as well as silicates) inside the steel are pressed into thin sheets, and delamination (sandwich) occurs. The delamination greatly deteriorates the tensile properties of the steel in the thickness direction, and it is possible that interlaminar tearing occurs when the weld is shrunk. The local strain induced by weld shrinkage often reaches several times the yield point strain, which is much larger than the strain caused by the load;
2. Residual stress caused by uneven cooling. The residual stress is the internal self-phase equilibrium stress without external force. The hot-rolled steel of various sections has such residual stress. The larger the section size of the general section steel, the larger the residual stress. Although the residual stress is self-phase-balanced, it still has some influence on the performance of steel members under external force. Such as deformation, stability, fatigue and other aspects may have adverse effects.
Advantages and disadvantages of cold rolling:
Advantages: Fast forming speed, high output, and no damage to the coating, can be made into a variety of cross-section forms to meet the needs of the use conditions; cold rolling can make the steel produce a large plastic deformation, thereby improving the yield of the steel point.
1. Although there is no hot plastic compression in the forming process, there is still residual stress in the cross section, which inevitably affects the overall and local buckling characteristics of the steel;
2. The cold-rolled steel profile is generally an open section, so that the free torsional stiffness of the section is low. It is easy to twist when being bent, and it is prone to bending and torsion buckling when pressed, and the torsion resistance is poor;
3. Cold-rolled steel coil has a small wall thickness and is not thickened at the corners where the plates are joined. The ability to withstand localized concentrated loads is weak.
The difference between cold rolling and hot rolling
1. Cold-rolled steel allows partial buckling of the section, so that the bearing capacity of the bar after buckling can be fully utilized; and the hot-rolled steel does not allow local buckling of the section.
2, the residual stress of hot-rolled steel and cold-rolled steel is different, so the distribution on the cross-section is also very different. The residual stress distribution on the section of the cold-formed thin-walled steel is curved, while the residual stress distribution on the section of the hot-striped steel or welded steel is a film type.
3. The free torsional stiffness of hot-rolled steel is higher than that of cold-rolled steel, so the torsional properties of hot-rolled steel are better than those of cold-rolled steel.