Hot Rolled Steel VS Cold Rolled Steel

16 March 18

Sinowell Metal supplies China best hot rolled steel like hot rolled steel plate and cold rolled steel like cold rolled steel coils from China highest end steel mill.

Hot steel rolling and cold steel rolling are all the forming processes of steel plates or steel profiles.They have a great impact on the steel structures and properties. Steel rolling is mainly to hot rolling, cold rolling is usually used only for the production of small scale steel and thin plate and other precision steel.

Common Cold Rolled Steel and Hot Rolled Steel Classification:

Wire rod: diameter 5.5-40 mm, coil-shaped, all hot-rolled steel. After cold drawn then is belong to the cold rolled steel.

Round Bar: In addition to the size of the bright light outside the general are hot-rolled steel, there are forging material (surface forging traces).

Steel Strip: there have ones of hot-rolled and cold-rolled and cold rolled strip is generally thin.
Steel plate: the cold-rolled plate is generally thin, such as automotive steel; the hot-rolled plate is more common. It also has the similar thickness with the cold rolled, but the appearance is significantly different.

Angle steel: all hot rolling.

Steel pipe: welding \ hot rolling and cold drawing.

Channel and H-beam: hot-rolled.

Reinforcing steel wire: hot rolled material.

Hot Rolling: by definition, ingot or billet is difficult to deform at room temperature, hard to be processed, typically heated to 1100 ~ 1250 ℃ to roll, this is called hot-rolled rolling process.

Hot rolling termination temperature is generally 800 ~ 900 ℃, generally after cooled in air, and thus the hot-rolled state is equivalent to have the normalizing treatment.
Most of the steel is rolled by hot rolling. Hot-rolled state of steel, due to high temperature, the surface will happen a layer of oxide scale, which has a certain degree of corrosion resistance. It can be stored in the open air.
But this layer of iron oxide skin also makes hot rolled steel surface rough, large size fluctuations, so the requirements of the surface smooth, accurate size, good mechanical properties of steel, will use hot-rolled semi-finished or finished steel as raw materials to produce cold rolled steel.

Hot Rolled Steel advantages:

It can destroy the cast structure of steel ingot, refine steel grains, eliminate micro-structure’s defects to make the steel organization compacted, mechanical properties improved.
This improvement is mainly reflected in the rolling direction so that the steel is no longer certain extent isotropic body. And bubbles, cracks and loosely formed structures during the casting process, can be welded together at the elevated temperature and pressure.

Hot-Rolled Steel Disadvantages:

1. After hot rolling, the nonmetallic inclusions (mainly sulfides and oxides, and silicates) inside the steel are pressed into flakes and will occur denomination. The steel denomination will deteriorate the tensile performance in the thickness direction significantly and may occur inter-layer tearing when the weld joint shrinking. The fractional pre-stressed strain caused by the weld joint shrinkage often reaches several times the yield strength, the strain is much larger than the load caused;

2. The residual stresses caused by uneven cooling. Residual stresses are the equilibrium stresses from the internals of the steel with the absence of an external force. All kinds of hot-rolled section steel with various cross-sections has such residual stresses. The bigger cross-sectional dimension of the steel, the larger of the residual stresses it has. Even the residual stress is self-balanced, but there still have some of the certain influence on the steel properties when an external forces onto it. It will adverse effects on steel deformation aspects, stability, anti-fatigue.

Cold Rolled Steel​​

Cold Rolling: Cold-rolling means at room temperature, with the roller pressure to press the steel, to change the steel shapes. Although the process will also make steel to heat up, but still called cold rolling. Specifically, cold rolled steel is with hot rolled coil as raw material, pickling to remove the oxide scale after the pressure processing, the finished cold rolling steel is called hard rolls.

General speaking, cold-rolled steel such as galvanized steel, color coated steel must be annealed, so the plasticity and elongation of steel will be better. It is widely used in automobiles, home appliances, hardware and other industries.

Cold rolled sheet has a certain surface finish, hand feels relatively smooth, mainly due to the picking process and treatment. Hot-rolled sheet general surface finish can not meet the requirements, so hot-rolled steel strip needs to be cold-rolled. Also, hot-rolled strip thickness of the thinnest, in general, is 1.0mm, cold rolling steel can reach 0.1mm.

Hot rolling is a rolling above the steel crystallization temperature point, and cold rolling is a rolling below the steel crystallization temperature point.

Cold rolling to change the shape of the steel is belong to continuous cold deformation, this process causes work hardening of the strength of the rolled hard, increase in hardness, tough plastic index decreases. For use by the terminal application, Cold rolling deteriorates stamping properties. The product is suitable for simple deformation of the parts.

Cold Rolled Steel Advantages:

Forming fast, huge production quantities, and does not damage the coatings, can be made into a variety of cross-section form to meet the needs of the steel use of conditions; cold rolling can produce a large plastic deformation for steel, thereby increasing the yield of steel.

Cold Rolled Steel Disadvantage:

1.Although the molding process has not been hot plastic compression, still residual stress within the cross-section will inevitably have an impact on the overall and fractional buckling resistance of the steel.

2.Cold-rolled steel profile is generally open cross-section, making the cross section of the free torsional stiffness is low. When prone to bending torsional, bending and torsion prone to buckling when compressed, poor torsional property.

3, Cold rolled steel is with a thin wall thickness, at the corner of the adapter plate and no thickening, the ability to withstand concentrated localized strength is weak.

In summary

The difference between hot rolling and cold rolling are mainly rolling process temperature. “Cold” is room temperature, “hot” is a high temperature

From the point of view of metallurgy, the boundaries of cold rolling and hot rolling should be distinguished by re-crystallization temperature. Rolling below the re-crystallization temperature is cold rolling, rolling higher than the re-crystallization temperature is hot rolling. The re-crystallization temperature of the steel is 450 ~ 600 ℃.

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