Characteristics of Friction in Rolling Process

During the rolling process, the relative motion between the rolling mill and the rolled piece generates resistance that hinders the flow of metal particles on the contact surface, which is called external friction. Its resistance is called frictional resistance or friction force, and the direction of friction force is opposite to the direction of motion. When the rolled piece undergoes plastic deformation, the friction caused by the relative motion (slip) of particles inside the metal M is called internal friction. The internal friction of solids is a direct consequence of the forced motion of whole molecules, which, in equilibrium, are closely spaced and exhibit a strong mutual attraction and repulsion between each other. Internal friction causes internal shear of the metal body and leads to internal heating. So far, the research on internal friction of metal materials is still insufficient, and the data is also very scarce. Therefore, the friction discussed in the rolling process usually refers to the external friction between the rolling mill and the rolled piece.

Compared with general mechanical motion, friction in the rolling process should also be based on general friction theory, as the physical properties of the contact surface are the same. However, there are differences between the two. Friction in metal rolling has the following characteristics:

(1) Internal and external friction coexist. During the rolling process, due to plastic deformation of the metal, internal and external friction coexist and interact with each other. The manifestation of internal friction is the generation of deformation heat, while only external friction exists in mechanical motion.

(2) High contact pressure. During metal rolling, the contact surface is subjected to high contact pressure. During hot rolling, the contact unit pressure reaches 50-500MPa. During cold rolling, it can reach 500-2500MPa. In the operation of machinery, the pressure on general heavy-duty bearings is only 20-50MPa

(3) There are many factors that affect friction. Contact friction stress is a function of the stress state of the metal in the deformation zone, the geometric parameters of the deformation zone, and the external rolling process conditions (temperature, speed, degree and mode of deformation, etc.). For example, frictional stress is a function of the coordinate points of the contact surface. During hot rolling, the closer it is to the neutral plane in the deformation zone, the greater the contact frictional stress; The friction stress of thin parts is greater than that of thick parts; The frictional stress at high temperatures is generally greater than that at low temperatures.

(4) The condition and properties of the contact surface are constantly changing. The contact between operating mechanical parts belongs to the elastic variation range. The overall parts will not undergo plastic deformation, only a small amount of new surface will be generated due to wear. During the metal rolling process, the rolled piece undergoes plastic deformation, and the contact surface continuously expands and updates (internal particles transfer to the surface). In addition, after the surface oxide layer is damaged, the new metal surface will be exposed, which will cause changes in the contact surface condition, microstructure, and properties. Especially at high temperatures, for example, steel heated to 950-1160 ℃, aluminum

At 350-650 ℃, the surface of the workpiece oxidizes, and the composition and properties of the oxide layers on the surface (for example, the oxide layer of steel has three types of oxide layers: Fe2O3, Fe3O4, and FeO) are not consistent, which will change the contact friction stress. Usually, high-temperature oxides can reduce friction and provide lubrication; At room temperature, the properties of oxides are relatively hard and brittle. After the oxide film is broken during processing, it plays a role in abrasive grinding. During cold working, work hardening can cause changes in the metal structure and properties, as well as changes in the friction condition of the contact pair.

In short, friction always exists in the rolling process, and its effect, like the force of gravity on nature, sometimes requires it and sometimes tries to avoid it.