Features of friction during rolling

During the rolling process, the relative movement between the roll and the rolled piece produces resistance that hinders the flow of metal particles on the contact surface, which is called external friction. The resistance is called friction resistance or friction, and the direction of friction is opposite to the direction of movement. When the rolled piece undergoes plastic deformation, the friction caused by the relative movement (slip) of the internal particles of the gold M is called internal friction. The internal friction of a solid is the direct consequence of the forced movement of the whole molecules. These molecules are closely spaced in equilibrium and show a strong mutual attraction and repulsion between each other. Internal friction causes internal shearing of the metal body and causes internal heating. So far, the research on internal friction of metal materials has not been enough, and the data is also very scarce. Therefore, the friction discussed in the rolling process usually refers to the external friction between the roll and the rolled piece.

The friction in the rolling process is compared with the general mechanical motion. Because the physical properties of the contact surface are the same, the study of the friction in the milking process should also be based on the general friction theory. However, there are differences between the two. The friction in metal rolling has the following characteristics:

(1) Internal and external friction exists at the same time. In the rolling process, due to the plastic deformation of the metal, internal and external friction exists at the same time and interacts. The manifestation of internal friction is deformation heat, while only external friction exists in mechanical motion.

(2) The contact pressure is high. When metal is rolled, the contact surface is subjected to higher contact pressure. During hot rolling, the contact unit pressure reaches 50-500MPa. It can reach 500-2500MPa during cold rolling. In operating machinery, the pressure on the heavy-duty bearing is generally only 20-50MPa_o

(3) There are many factors that affect friction. The 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, deformation degree and deformation method, etc.). For example, the friction stress is a function of the coordinate points of the contact surface. During hot rolling, the closer to the neutral surface of the deformation zone, the greater the contact friction stress; the friction stress of thin parts is greater than that of thick parts; the friction stress at high temperature is generally higher than that at low temperature The time is bigger.

(4) The condition and nature of the contact surface are constantly changing. The contact between moving mechanical parts belongs to the range of elastic change. The whole part will not undergo plastic deformation, only a small amount of new surface will be produced due to wear. In the metal rolling process, the rolled piece undergoes plastic deformation, and the contact surface is continuously expanded and renewed (the internal particles are transferred to the surface). In addition, after the surface oxide film layer is destroyed, the new metal surface is exposed, which will cause changes in the contact surface condition, organization and performance. Especially at high temperature, for example, when steel is heated to 950-1160℃, aluminum

At 350-650℃, the surface of the workpiece is oxidized, and the oxide layers on the surface are composed of oxides (for example, the oxide layer of steel has Fe₂O₃, Fe_{3< /sub>O4 and FeO three oxide layers) are inconsistent with their properties, which will change the contact friction stress. Generally, high temperature oxides can reduce friction and have a lubricating effect; while room temperature oxides are harder and brittle in nature. After the oxide film is broken during processing, it can play an abrasive grinding role. During cold working, due to work hardening, the structure and properties of the metal will change, and the friction condition of the contact pair will also change.}

In short, friction always exists in the rolling process, and the effect of friction is like the effect of universal gravitation on the natural world. Sometimes it is needed, and sometimes it is avoided as much as possible.