I. Advantages and disadvantages of hot rolling and cold rolling
Advantages of hot rolling
Fast forming speed and high yield: the hot rolling process adopts continuous and automatic production mode, which can complete the forming of steel quickly and with high production efficiency.
No damage to the coating: the steel surface coating is not easy to be damaged during the hot rolling process, maintaining a better surface quality.
Can be made into a variety of cross-section form: hot rolling can be adapted to different cross-section needs, to produce a variety of shapes of steel, to meet the diverse conditions of use.
Significantly reduce energy consumption, reduce costs: hot rolling when the metal plasticity is high, low deformation resistance, can significantly reduce the energy consumption of metal deformation.
Improve the processing performance of metals and alloys: hot rolling can break the casting state of coarse grains, reduce or eliminate casting defects, improve the processing performance of alloys.
Disadvantages of hot rolling
Residual stresses exist within the cross-section: during hot rolling, residual stresses may exist within the steel, which have an effect on the overall and local buckling properties of the steel.
Poor torsional resistance: hot rolled steel is generally open cross-section, low free torsional stiffness, easy to torsion when bending, poor torsional resistance.
Weak ability to withstand localised concentrated loads: the wall thickness of hot rolled steel is small, and it is not thickened at the corners where the plates are connected, so the ability to withstand localised concentrated loads is weak.
Difficulty in controlling the thickness dimension of products: the accuracy of controlling the thickness dimension of hot rolled products is relatively poor, and the surface is rougher than that of cold rolled products.
Uneven organisation and properties: hot rolling cannot control the required mechanical properties of the products very accurately, and the organisation and properties of the hot rolled products cannot be uniform.
Advantages of cold rolling
Fast forming speed and high yield: Cold rolling also has fast forming speed and high yield.
No damage to the coating: the surface coating of the steel is also not easily damaged during the cold rolling process.
Variety of cross-sections: Cold rolling also produces a wide range of cross-sections.
Increased yield point: Cold rolling produces large plastic deformations in the steel, thus increasing the yield point of the steel.
Good surface quality: the surface quality, appearance and dimensional accuracy of cold rolled steel plate are better than that of hot rolled plate.
Disadvantages of cold rolling
Residual stresses: residual stresses may also exist within the steel during cold rolling.
Poor torsional performance: cold rolled steel style is generally open section, the free torsional stiffness of the section is low, poor torsional performance.
Weak ability to withstand localised concentrated loads: the wall thickness of cold rolled formed steel is small, and the ability to withstand localised concentrated loads is weak.
In summary, hot rolled and cold rolled have their own advantages and disadvantages, the choice of which processing mode mainly depends on the specific needs and conditions of use.
II. the difference between hot rolled steel and cold rolled steel
1 formation
Hot rolled steel: slab as raw material, after heating by the roughing unit and finishing unit made of strip.
Cold rolled steel: hot rolled steel coils as raw materials, after pickling to remove the oxide skin for cold rolling, the finished product is rolled hard coil.
2 Properties
Hot rolled steel:
Strength: not very high, but enough to meet the needs of general use.
Plasticity, weldability: better, so hot rolled steel is more commonly used.
Appearance of colour: not bright, the surface may be with an oxide layer or iron tetraoxide ooze layer.
Cold rolled steel:
Strength: higher.
Toughness, weldability: relatively poor, the material is hard and brittle.
Surface quality: because the cold rolling process will be surface finishing, so the surface quality of cold rolled steel is usually better than hot rolled steel, bright surface.
3 Process
Hot rolled steel: ironmaking → steelmaking → continuous casting (or mould casting) → hot rolling (out of hot rolled products). Hot rolling process requires high-temperature heating of the material, generally to be heated to the recrystallisation temperature above the rolling, in order to make the steel internal recrystallisation, better material properties.
Cold rolled steel: on the basis of hot rolled steel, further cold rolling process, that is, ironmaking → steelmaking → continuous casting (or mould casting) → hot rolling (out of hot rolled products) → cold rolling (out of cold rolled products). Cold rolling process requires larger mill power, and rolling efficiency is low, usually also need to carry out intermediate annealing to eliminate work hardening.
4 Appearance and surface quality
Hot rolled steel: usually black or dark grey in colour, the surface may have an oxide or iron tetraoxide ooze layer.
Cold rolled steel: better surface quality, usually metallic colour, smooth and bright.
In summary, there are significant differences between hot rolled steel and cold rolled steel in terms of formation, properties, process and appearance and surface quality. Hot rolled steel has better plasticity and weldability, but the surface quality is relatively poor; while cold rolled steel has higher strength and better surface quality, but toughness, weldability is relatively poor. Therefore, in practical applications, it is necessary to choose the appropriate type of steel according to specific needs.
III. Main uses of hot rolled steel and cold rolled steel
The main uses of hot-rolled steel
Structural steel:
It is mainly used in the production of steel structural parts, bridges, ships and vehicles. Hot rolled steel is widely used in large building structures due to its good workability and weldability.
Weather-resistant steel:
Hot rolled steels with special elements (e.g. P, Cu, C, etc.) have good resistance to corrosion and atmospheric corrosion, and are commonly used in the production of containers, special vehicles, and building structures.
Steel for automotive structures:
Used in the production of automotive FRAME, WHEEL, etc., as it has good stamping properties and welding properties.
Hot rolled special steels:
Carbon steel, alloy steel, tool steel for general mechanical structure, used in the production of various mechanical parts after heat treatment works.
Steel plates for steel pipes:
Used for the production of high-pressure gas pressure vessels filled with LPG, acetylene gas and various gases with an internal volume of 500L or less, due to its good processing performance and compressive strength.
Stainless steel plate:
Mainly used in food industry, surgical equipment, aerospace, petroleum, chemical and other industries, because of its good corrosion resistance.
The main uses of cold rolled steel
Construction field:
Used in the production of building structural skeleton members, columns, beams, purlins, wall panels, etc. The high strength and good forming properties of cold rolled steel make it an important application in building structure.
Automotive Manufacturing:
Used for automobile shell, body structure, bonnet, door, body skeleton, automobile suspension system, etc. Cold rolled steel has excellent hardness, toughness, strength, formability and corrosion resistance, in line with the requirements of automotive manufacturing on steel performance.
Machinery manufacturing field:
It is widely used in the manufacture of various machines, instruments, gauges, etc. For example, aluminium alloy cold rolled sheet is widely used in the manufacture of automobiles, trains, aircraft, ships, satellites and other means of transport, and alloy steel cold rolled sheet is used in the manufacture of boilers, pressure vessels and so on.
Power industry:
Used in the manufacture of power equipment shells, such as large generators, transformers, capacitors and so on. Cold rolled steel requires high content of porosity and inclusions in the material, because the safety and reliability of power equipment is directly related to the stable operation of the power grid.
Home appliance industry:
In the manufacture of refrigerators, washing machines, air conditioners and other electrical appliances shell or parts, cold rolled steel has the advantages of low cost, good corrosion resistance, high strength, etc.
