1、The surface of the cold-rolled sheet has a certain degree of glossiness and feels smooth to the touch, similar to the common steel cup used for drinking water.
2、If the hot-rolled plate is not pickled, the surface is similar to that of many ordinary steel plates on the market. The rusted surface is red, and the unrusted surface is purple-black (iron oxide scale).
The performance advantages of cold-rolled plates and hot-rolled plates are：
(1) Higher precision, the thickness difference of cold-rolled strip does not exceed 0.01~0.03mm.
(2) The size is thinner, the thinnest cold-rolled steel strip can reach 0.001mm; the thinnest hot-rolled steel strip can now reach 0.78mm.
(3) The surface quality is better, and cold-rolled steel sheets can even produce mirror-like surfaces; while the surface of hot-rolled sheets has defects such as oxide scale and pitting.
(4) Cold-rolled steel can adjust its performance such as tensile strength and process performance such as stamping performance according to user requirements.
Cold rolling and hot rolling are two different steel rolling technologies. As the name implies, cold rolling is the rolling of steel at room temperature. This kind of steel has high hardness. Hot rolling is the rolling of steel at high temperature.
Details
Hot rolled plates have low hardness, are easy to process and have good ductility.
Cold-rolled plates have high hardness and are relatively difficult to process, but they are not easy to deform and have high strength.
Hot-rolled plates have relatively low strength and poor surface quality (oxidation and low finish), but good plasticity, and are generally medium-thick plates. Cold-rolled plates have high strength, high hardness, and high surface finish, and are generally thin plates that can be used as stamping plates.
The mechanical properties of hot-rolled steel plates are far inferior to those of cold-processed steel plates, and are also inferior to those of forged steel plates, but they have better toughness and ductility.
Cold-rolled steel plates have a certain degree of work hardening, so they have low toughness, but can achieve a good yield strength ratio, and are used to cold-bend parts such as spring leaves. At the same time, since the yield point is closer to the tensile strength, there is no foreseeability of danger during use, and accidents are prone to occur when the load exceeds the allowable load.
By definition, steel ingots or billets are difficult to deform and process at room temperature. They are generally heated to 1100-1250℃ for rolling. This rolling process is called hot rolling. Most steel products are rolled by hot rolling. However, because iron oxide scale is easily generated on the surface of steel at high temperature, the surface of hot-rolled steel is rough and the size fluctuates greatly. Therefore, steel products with smooth surface, precise size and good mechanical properties are produced by cold rolling with hot-rolled semi-finished products or finished products as raw materials. Rolling at room temperature is generally understood as cold rolling. From the perspective of metallurgy, the boundary between cold rolling and hot rolling should be distinguished by the recrystallization temperature. That is, rolling below the recrystallization temperature is cold rolling, and rolling above the recrystallization temperature is hot rolling. The recrystallization temperature of steel is 450-600℃.
Hot rolling, as the name implies, has a high temperature for the rolled piece, so the deformation resistance is small and a large deformation amount can be achieved. Taking the rolling of steel plates as an example, the thickness of the continuous casting billet is generally around 230mm, and after rough rolling and finishing rolling, the final thickness is 1~20mm. At the same time, due to the small width-to-thickness ratio of the steel plate, the dimensional accuracy requirements are relatively low, and the plate shape problem is not easy to occur, and the convexity is mainly controlled. For those with requirements for organization, it is generally achieved through controlled rolling and controlled cooling, that is, controlling the start rolling temperature of finishing rolling, the final rolling temperature and the curling temperature to control the microstructure and mechanical properties of the strip.
Cold rolling generally does not involve a heating process before rolling. However, due to the small thickness of the strip, it is easy to have problems with the shape of the strip. Moreover, the finished product is cold rolled, so many complicated processes are used to control the dimensional accuracy and surface quality of the strip. The cold rolling production line is long, with many equipments and complex processes. As users' requirements for the dimensional accuracy, shape and surface quality of the strip increase, the control models, L1 and L2 systems and shape control methods of the cold rolling unit are more than those of hot rolling. Moreover, the temperature of the rolls and the strip is also one of the more important control indicators.
For cold-rolled products and hot-rolled products, the difference is the previous process and the next process. Hot-rolled products are the raw materials of cold-rolled products. Cold-rolling is to put the hot-rolled steel coils that have been pickled on the machine using a roller mill. The rolling process is cold processing and forming. It mainly rolls thick hot-rolled plates into thin cold-rolled plates. Usually, for example, 3.0mm hot-rolled plates can be rolled on the machine to produce 0.3-0.7mm cold-rolled coils. The main principle is to use the extrusion principle to force deformation.
