Why is there a distinction between hot rolled steel and cold rolled steel and what is the difference? - Shandong Jiugang Tisco Steel Co., Ltd.

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Why is there a distinction between hot rolled steel and cold rolled steel and what is the difference?

Hot rolling and cold rolling are both processes for forming steel sheets or sections, and they have a significant impact on the organisation and properties of steel.

The rolling of steel is mainly hot-rolled, cold-rolled is usually only used to produce small sections and thin plates of precise size.

 

Common steel cold and hot rolling situations: Wire rod: 5.5 – 40 mm diameter, in coils, all hot rolled material. After cold drawing they are cold drawn.

Round steel: all hot-rolled steel, except bright steel, which is of precise dimensions, but also forged steel (with traces of forging on the surface).

Strips: both hot rolled and cold rolled, cold rolled steel is generally thinner.

Steel plates: cold rolled plates are generally thinner, such as those used for automobiles; hot rolled medium thick plates are more common and are available in similar thicknesses to cold rolled, with a distinctly different appearance.

Angles: all hot-rolled.

Steel pipes: both welded hot-rolled and cold-drawn.

Channels and H-beams: hot-rolled.

Reinforcing bars: hot rolled.

Hot rolled steel plate

By definition, ingots or billets are difficult to deform at room temperature and are not easily processed, and are generally heated to 1100 to 1250°C for rolling, a rolling process called hot rolling.

The termination temperature of hot rolling is generally 800 to 900°C, after which it is generally cooled in air, and thus the hot-rolled state is equivalent to normalizing treatment.

Hot-rolled state of delivery of steel, due to the high temperature, the surface of the generation of a layer of iron oxide, thus having a certain corrosion resistance, can be stored in the open.

However, this layer of iron oxide also makes the surface of hot-rolled steel rough, size fluctuations, so the requirement for a clean surface, accurate size, good mechanical properties of steel, to use hot-rolled semi-finished or finished products as raw materials and then cold-rolled production.

Advantages.

Fast forming, high yield, and no damage to the coating, can be made into a variety of cross-sectional forms to suit the conditions of use; cold rolling can produce a large plastic deformation of the steel, thus increasing the yield point of the steel.

Disadvantages:

1. Although the forming process does not go through the hot state plastic compression, but there are still residual stresses in the cross-section, the overall and local yielding characteristics of the steel is bound to have an impact;

2. The cold rolled steel style is generally an open section, making the free torsional stiffness of the section lower. In the bending prone to twisting, bending and torsional buckling when pressed, poor torsional performance;

3. The wall thickness of cold rolled formed steel is small and not thickened at the corners where the plates are joined, which makes the ability to withstand localised concentrated loads weak.

Cold rolled steel plate

Cold rolling is a rolling method that uses the pressure of rolls to squeeze steel at room temperature to change the shape of the steel. Although the process also heats up the steel, it is still called cold rolling. To be more specific, cold rolling uses hot rolled steel coils as raw material, and after pickling to remove oxidation skin, the finished product is rolled hard coils.

General cold-rolled steel plate such as galvanized, colour steel sheets are subject to annealing, so plasticity and elongation is also better, widely used in automotive, home appliances, hardware and other industries. The surface of cold-rolled sheet has a certain degree of finish and is smooth to the touch, mainly due to the work of pickling. Hot rolled sheet generally does not meet the surface finish requirements, so the hot rolled steel strip needs to be cold rolled, there are hot rolled steel strip thickness is generally the thinnest in 1.0mm, cold rolling can reach 0.1mm. hot rolling is the crystallization temperature point above the rolling, cold rolling is the crystallization temperature point below the rolling.

Cold rolling is a continuous cold deformation of the shape of the steel, and the cold work hardening caused by this process makes the strength and hardness of the rolled hard coil rise and the toughness index fall.

For end use, cold rolling deteriorates the stamping properties and the product is suitable for simply deformed parts.

Advantages:

It can destroy the casting organisation of the ingot, refine the grain of the steel and eliminate microstructural defects, thus making the steel organisation dense and improving the mechanical properties. This improvement is mainly 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 together under high temperature and pressure.

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