Characteristics and differences between hot rolling and cold rolling

What is hot rolling and cold rolling?

1. Hot rolling

Using continuous casting slabs or blooming slabs as raw materials, heating them in a walking furnace, removing phosphorus with high-pressure water, and then entering the rough rolling mill. After the final rolling, it goes through laminar cooling (computer-controlled cooling rate) and coiling by a coiler to become straight curls. The head and tail of the straight hair curler are often tongue-shaped and fish-tail-shaped, with poor thickness and width accuracy, and the edges often have defects such as wave shape, folded edge, and tower shape. The coil weight is heavier, and the inner diameter of the steel coil is 760mm. (Generally used in the pipe making industry.) After the straight hair roll is processed by cutting head, tail cutting, edge trimming and multi-pass straightening, leveling and other finishing lines, then cutting or re-rolling, it becomes: hot rolling Steel plates, flat hot rolled steel coils, slitting strips and other products. If the hot rolled finishing coil is pickled to remove oxygen

By definition, the steel ingot or billet is difficult to deform at room temperature and difficult to process. Generally, it is heated to 1100-1250 ℃ for rolling. This rolling process is called hot rolling.

The termination temperature of hot rolling is generally 800 to 900 ° C, and then it is generally cooled in air, so the hot rolling state is equivalent to normalizing treatment. Most steels are rolled by the hot rolling method. Due to the high temperature, the steel delivered in the hot-rolled state has a layer of iron oxide scale on the surface, so it has certain corrosion resistance and can be stored in the open air. However, this layer of iron oxide scale also makes the surface of the hot-rolled steel rough and the size fluctuates greatly. Therefore, the steel with smooth surface, accurate size and good mechanical properties is required, and hot-rolled semi-finished products or finished products are used as raw materials for cold rolling production. And after oiling, it becomes a hot-rolled pickling coil.

2. Cold rolling

Using hot-rolled steel coil as raw material, after pickling to remove oxide scale, cold continuous rolling is carried out, and the finished product is rolled hard coil. The cold work hardening caused by continuous cold deformation increases the strength, hardness and toughness and plastic index of rolled hard coil. , the stamping performance deteriorates, and it can only be used for parts with simple deformation. Hard-rolled coils can be used as raw materials for hot-dip galvanizing plants, because hot-dip galvanizing units are equipped with annealing lines. The weight of the rolled hard coil is generally 6~13.5 tons, and the inner diameter of the steel coil is 610mm. Generally, cold continuous rolled sheets and coils should be de-annealed by continuous annealing (CAPL unit) or bell furnace to eliminate cold work hardening and rolling stress and achieve the mechanical properties specified in the standard. The surface quality, appearance and dimensional accuracy of cold-rolled steel sheets are better than those of hot-rolled steel sheets.

Cold rolling refers to the rolling method of changing the shape of the steel by extruding the steel under the pressure of the roll at room temperature. Although the processing also heats up the steel sheet, it is still called cold rolling. Specifically, the hot-rolled steel coil for cold rolling is used as the raw material, which is subjected to pressure processing after pickling to remove the oxide scale, and the finished product is a hard-rolled coil. The surface of the cold-rolled sheet has a certain smoothness, and the hand feels smoother, mainly due to the pickling. The change of the shape of steel by cold rolling belongs to continuous cold deformation. The cold work hardening caused by this process increases the strength, hardness and toughness and plastic index of the rolled hard coil.

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

The main difference between hot rolling mill and cold rolling mill

Cold-rolled section steel allows local buckling of the section, so that the bearing capacity of the member after buckling can be fully utilized, while hot-rolled section steel does not allow local bending in the section.

The causes of residual stress in hot-rolled steel and cold-rolled steel are different, so the cross-section distribution is also quite different. The residual stress distribution on the cold-formed thin-walled steel section is a bending type, while the residual stress distribution on the hot-rolled or welded section steel section is a thin-film type.

The free torsional stiffness of hot-rolled steel is higher than that of cold-rolled steel, so its torsional performance is better than that of cold-rolled steel.

Advantages and disadvantages of hot rolling steel and reversing cold mill

1.hot rolled

Advantages: It can destroy the ingot structure, refine the steel grains, eliminate microstructure defects, make the steel structure dense, and improve the mechanical properties of the steel. It is mainly manifested in the upward direction of rolling, so that the steel is no longer an isotropic body; bubbles, cracks and looseness are generated during casting, and can also be welded under high temperature and high pressure.

Disadvantages: After hot rolling, the non-metallic inclusions (mainly sulfides and oxides, as well as silicates) inside the steel are pressed into thin sheets, and the phenomenon of delamination (interlayer) occurs. The layered structure deteriorates the tensile properties of the steel in the thickness direction and may lead to interlaminar tearing when the weld shrinks. During the welding process, the local strain due to welding shrinkage often reaches several times the yield point strain, which is far greater than the strain caused by the load;

Residual stress due to uneven cooling. This stress is the stress of internal self-phase equilibrium under the action of no external force. This kind of residual stress will be generated by hot-rolled section steel of various sections. Generally, the larger the section size of the section steel, the greater the residual stress. Although the residual stress is self-equilibrium, it still affects the performance of the component under external force. For example, it has adverse effects on deformation, stability, and fatigue resistance.

2.Cold rolling

Advantages: fast forming speed, high output, and no damage to the coating, can be made into various cross-sectional forms to meet the needs of use conditions; cold rolling can produce large plastic deformation of steel, thereby improving the yield of steel point.

Disadvantages: Although thermoplastic compression is not performed during the forming process, there is still residual stress on the section, which will inevitably affect the overall and local buckling characteristics of the steel;

Cold-rolled sections are mostly open sections, and their free torsional stiffness is low. The wall thickness of cold-rolled forming steel is small, the corners of the connection do not need to be thickened, and the ability to bear local concentrated loads is weak, so it is prone to torsional deformation and bending-torsional buckling when it is bent.