There are four main factors that affect the rolling during the rolling process:
1. The bite of the rolling piece
1. Biting conditions In order to realize the rolling process, the rolling piece must be bitten into the roll first, and then the metal can fill the gap of the roll for rolling. Practice has proved that the bite of metal and the establishment of rolling process cannot be realized under any circumstances, so it is necessary to study the conditions of the bite of the roll into the rolling stock.
At the moment of bite of the rolling stock, each roll will apply two forces to the rolling stock. One is the normal force (radial force) N perpendicular to the contact between the roll and the metal, and the other is the friction force T at the tangent point between the roll and the surface of the rolling piece. The direction of the vertical component of force N and force T is the same, so that The metal is compressed and deformed, and the horizontal components of force N and force T are in opposite directions. Obviously, according to the action of these two forces, it can be seen that:
(1) If >, it is impossible for the roll to bite the rolling piece, and the rolling process cannot be realized.
(2) If = , it is in a state of equilibrium, and the rolling piece is still impossible to be naturally bitten.
(3) If <, the roll can bite the rolling piece. Therefore, if the inertial force during biting is not considered, to achieve biting, the following conditions must be met: > After the rolling piece is bitten and filled between the rolls, the contact area between the rolling piece and the rolling piece gradually increases, and the contact area between the rolling piece and the rolling piece is gradually increased. The point of action of the resultant pressure also moves inward gradually, and the relationship between the maximum bite angle and the friction angle also changes accordingly.
2. Factors affecting the bite of rolled pieces
⑴Influence of roll diameter and reduction: When the plate thickness is constant, the larger the roll diameter, the smaller the bite angle and the easier it is to bite.
(2) Influence of the shape of the rolled piece: Because the shape of the front end of the rolled piece is different, it has a great relationship with the difficulty of biting. When the front end of the rolling stock is larger than the rear end, it is not conducive to biting, and when the front end of the rolling stock is smaller than the rear end, especially if the ends are pointed or boat-shaped, it is conducive to biting. At the moment of starting to bite, the position of the contact point between the rolling piece and the roll and the size of the contact surface are different. Obviously, the more the contact point moves inward and the larger the contact area, the more conducive to the bite. For easy bite, the front (or rear) of the ingot is often made into a pointed wedge, round or oval shape.
(3) The influence of the surface state of the roll on the bite The rougher the surface of the roll (for example, the common material roll after working several shifts), the larger the friction coefficient is, which is conducive to the bite. Conversely, it is difficult to bite into the rolling stock with a smooth surface (such as a cold-rolled polishing roll).
⑷ The effect of rolling speed on bite The increase of the rolling speed is not conducive to the bite of the rolling piece, and the reduction of the rolling speed is conducive to the bite of the rolling piece. The reason why the rolling speed affects the bite is that, on the one hand, the increase of the rolling speed reduces the friction coefficient between the roll and the rolling piece, making it difficult to bite, and on the other hand, the increase of the rolling speed hinders the rolling piece. Inertia force of bite. In some rolling mills, in order to maintain the high productivity of the rolling process, but avoid the adverse effect of the bite caused by increasing the rolling speed, the rolling method with adjustable speed is usually used. During the biting period, the roll speed is reduced by one called the biting speed. After the rolling piece is bitten, the roll speed is increased to carry out normal rolling.
2. Slip forward
Forward slip During rolling, the speed of metal thrown from the roll is greater than the linear velocity of the circumference of the roll. This phenomenon is called forward slip. The forward slip value can be expressed as:
S – forward slip value
V——Circumferential speed of the roll
V1——The speed of the rolling stock leaving the roll
During cold rolling, according to different conditions such as processing rate, tension, friction coefficient, etc., the forward slip value is generally within the range of 0-6%.
1. Determination of the forward slip value
In the experimental method, the notch method is relatively easy to do, that is to notch the surface of the roll, and the distance is L. , the indentation distance on the surface of the rolled piece after rolling is L1, then the forward slip value (L1-L.)/L can be calculated according to the following formula. ×100%, when this method is used to measure the front slip value, it is not only accurate and simple, so it is widely used, but its disadvantage is that it can only measure the sliding of the surface, not the internal sliding of the metal.
2. The main factors that affect the forward slip
During the rolling process, there are many factors that affect the front slip. According to the theoretical calculation formula and many experiments, the main factors affecting the front slip are: reduction, roll diameter, friction coefficient, front and rear tension, rolling speed and rolling stock. Width, rolling temperature and metal type affect the forward slip in the form of friction coefficient.
The influence of these main factors on the front slip is as follows: with the increase of reduction, roll diameter, friction coefficient and front tension, the front slip value increases, and conversely, the front slip value increases with the increase of post tension and rolling speed. Decrease, the front slip value also changes with the change of the width of the rolling stock, but when the width of the rolling stock reaches a certain limit and continues to increase the width, the front slip value does not change.
Spread refers to the size change of the rolling stock along the width direction during the rolling process, also known as transverse spread. Spreading is a phenomenon of transverse deformation of rolling stock. There are two ways to express the spread, the absolute spread and the relative spread. Absolute spread refers to the difference between the width before and after rolling, and relative spread refers to the ratio of the difference between the width before and after rolling and the width before rolling.
1. The practical significance of friction in the rolling process
When cold rolling sheets, it is generally desirable to reduce the coefficient of friction, because under such rolling conditions, it is not the maximum bite angle that limits the smooth progress of the rolling process, but the maximum pressure allowed by the rolling mill. At the same time, the friction coefficient has an important influence on the size and distribution of the unit pressure. The friction force generated by the friction coefficient requires additional work to overcome it. The increase of the friction coefficient increases the deformation resistance of the metal, which increases the energy consumption of deformation. . When calculating the rolling force, the average value of the friction coefficient along the bite arc should be accurately determined so that the rolling force can be calculated correctly.
2. Friction coefficient under different rolling and lubrication conditions
Although the coefficient of friction is so important, it is quite difficult to directly measure the coefficient of friction, because the coefficient of friction is related to many factors, such as roll surface state, lubrication conditions, rolling pressure, rolling temperature, etc. It should be pointed out that under the same conditions, different measurement methods often get conflicting results, so for specific rolling conditions, when choosing the friction coefficient, you must pay attention to the original conditions of the test data.