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Why should we divide the hot-rolled steel and cold rolled steel?

Date of release:2018-07-05 Author: Click:

Hot rolling and cold rolling are processes for forming steel plates or profiles. They have a great influence on the structure and properties of steel.

The rolling of steel is mainly based on hot rolling. Cold rolling is usually only used for producing precision steel such as small steel and thin plate.

Cold and hot rolling of common steel:

Wire: 5.5-40 mm in diameter, coiled, and all hot rolled. After cold drawing, it belongs to the cold drawn wood.

Round steel: in addition to the precision bright lumber, it is usually hot rolled, but also forged (with forging marks on the surface).

Strip steel: cold rolled and cold rolled products are generally thin.

Steel plate: cold rolled plate is generally thin, such as automobile plate; hot rolled plate is more, and has similar appearance with cold rolling, the appearance is obviously different.

Angle steel: all hot rolling.

Steel pipe: both hot rolling and cold drawing are available.

Channel steel and H type steel: hot rolling.

Steel: hot rolled material.

Hot-rolling

In terms of definition, steel ingot or billet is difficult to be deformed at normal temperature and is not easy to be processed. It is generally heated to 1100~1250 C to be rolled. This rolling process is called hot rolling.

The final temperature of hot rolling is generally 800~900 degrees, which is usually cooled in the air, so the hot rolling state is equivalent to normalizing.

Most of the steel is rolled by hot rolling. Due to the high temperature, the surface of the steel delivered in hot rolling condition has a layer of iron oxide on the surface, so it has certain corrosion resistance and can be stored in open air.

But this layer of iron oxide skin also makes the surface of hot rolled steel rough and the size fluctuates greatly, so the steel which requires smooth surface, accurate size and good mechanical properties should be cold rolled with hot rolled semi-finished products or finished products.

Advantage:

The molding speed, high production, and no damage to the coating can be made into a variety of cross section forms to meet the requirements of the use conditions; cold rolling can make a large plastic deformation of the steel, thus improving the yield point of the steel.

Shortcomings:

1. Although there is no thermal plastic compression during the molding process, the residual stress still exists in the section, which will inevitably affect the characteristics of the overall and local buckling of the steel.

2, the cold rolled steel type is usually open section, so that the free torsion stiffness of the section is low. Torsion is prone to occur during bending, and flexural torsional buckling is prone to occur under compression.

3. The thickness of cold rolled forming steel is small, and it is not thickened at the corner of plate joining.

cold rolling

Cold rolling refers to the rolling method of changing the shape of steel at the normal temperature by extruding steel with roller pressure. Although the process will also heat up the steel plate, it is still called cold rolling. To be specific, cold rolled hot-rolled coil is used as raw material to remove the oxide scale after pickling and then press it.

Generally cold rolled steel, such as galvanized and colored steel sheets, must be annealed, so the plasticity and elongation are good. It is widely used in automobile, household appliances, hardware and other industries. The surface of the cold rolled sheet has a certain degree of smoothness, and the handle feels smooth. Hot-rolled plate generally does not meet the requirements of surface finish, so hot rolled steel strip needs cold rolling, and hot-rolled steel strip is thinner in thickness than 1.0mm, and cold rolling can reach 0.1mm. Hot rolling is the rolling above the crystallization temperature point, and cold rolling is the rolling below the crystallization temperature point.

The change of cold rolled steel shape belongs to continuous cold deformation. The cold hardening caused by this process makes the strength and hardness rise and the toughness index decrease.

For terminal use, cold rolling makes the stamping performance deteriorate, and the product is suitable for simply deformed parts.

Advantage:

It can destroy the casting 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 direction of rolling, so that the steel is no longer isotropic to a certain extent; the bubbles, cracks and porosity formed during pouring can also be welded under the action of high temperature and pressure.

Shortcomings:

1. After hot rolling, the non-metallic inclusions (mainly sulphides and oxides, and silicates) inside the steel are compressed into thin sheets, and there is a phenomenon of stratification. The lamination makes the performance of steel pulling along the thickness direction greatly deteriorate, and it is possible to occur interlaminar tearing when the weld seam shrinks. The local strain induced by weld shrinkage often reaches the yield point strain several times, much larger than the strain induced by load.

2. Residual stress caused by inhomogeneous cooling. The residual stress is the internal self phase equilibrium stress without external force, and all kinds of hot rolled steel sections have this kind of residual stress. The larger the size of the general steel section, the greater the residual stress. Although the residual stress is self - balanced, it has certain effect on the performance of steel members under external force. Such as deformation, stability, anti fatigue and other aspects may have an adverse effect.

Summary:

The cold rolling and hot rolling are mainly the temperature of rolling process. "Cold" is the normal temperature, and "heat" is high temperature.

From the viewpoint of metallization, the boundary between cold rolling and hot rolling should be distinguished by recrystallization temperature. That is, the cold rolling is lower than the recrystallization temperature, and the rolling is higher than the recrystallization temperature. The recrystallization temperature of the steel is 450~600.

The main difference between hot rolling and cold rolling is:

1, appearance and surface quality: because the cold plate is obtained after the cold rolling process, and the cold rolling can also carry out some surface finishing so that the cold plate is on the surface quality.


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