What is the difference between hot rolled steel and cold rolled steel?
Conception of hot colled and cold colled
Continuous casting slab or slab is used as raw material, heated by step heating furnace, high-pressure water dephosphorization and then enters the rough rolled machine. The rough rolled material passes through cutting the head and tail, and then enters the finishing machine which carrys out by computer controlled rolled. After the final rolled, it is subjected to laminar cooling (computer controlled cooling rate) and the coiler is taken up to become a straight hair roll. The head and tail of the straight hair roll tend to be tongue-shaped and fish-tail-shaped, and the thickness and width are poorly accurate. The edge often has 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 coil is wound through cutting, trimming and multi-pass straightening, leveling and other finishing lines, then cutting or rewinding, it becomes hot rolled steel plates, flat hot rolled steel coils, slitting belts and other products. If the hot-rolled finishing rolls are pickled to remove scale and oiled, they are hot-rolled pickled rolls.
The hot-rolled steel coil is used as raw material, and the oxide scale is removed by pickling to be cold-rolled, and the finished product is rolled hard coil. The cold work hardening caused by continuous cold deformation causes the strength and hardness of the rolled hard roll to rise, and the toughness index decreases. The stamping performance deteriorates and can only be used for parts that are simply deformed. Rolled hard coil 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 steel coil is 610mm. Generally, the continuous rolled machine and coil should be subjected to annealing treatment (CAPL unit) or hood furnace to eliminate annealing work and rolled stress, and meet the mechanical properties specified in the standard. The surface quality, appearance and dimensional accuracy of cold rolled steel sheets are superior to those of hot rolled sheets.
Advantages and disadvantages of hot rolled
It can destroy the cast structure of the ingot, refine the grain of the steel, and eliminate the defects of the microstructure, so that the steel structure is compacted and the mechanical properties are improved. This improvement is mainly reflected in the rolled direction, so that the steel is no longer an isotropic body to some extent; bubbles, cracks and looseness formed during casting can also be welded under high temperature and pressure.
1. After hot rolled, 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 rolled:
Fast forming speed, high output, without damaging the coating, can be made into a variety of cross-section forms to meet the needs of the use conditions; cold rolled 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 section, which will inevitably affect 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. The cold-rolled steel has a small wall thickness, and there is no thickening at the corners where the plates are joined, and the ability to withstand localized concentrated loads is weak.
Difference between hot rolled and cold rolled
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 causes of residual stresses in hot-rolled and cold-rolled steels are 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 performance of hot-rolled steel is better than that of cold-rolled steel.
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