2. Rolling parameters: In the rolling deformation process, the compression in the thickness direction is the dominant deformation. When the thickness direction of the rolled piece is compressed by the roller, the metal will flow in the longitudinal direction and the transverse direction, but the deformation in the longitudinal direction always exceeds that in the transverse direction, because the friction on the roller surface always hinders the wide flow more than that in the longitudinal direction, that is to say, compared with the longitudinal direction, the transverse extension is always small, and the deformation index during rolling is mainly:
1) Absolute reduction: indicates the absolute change of the thickness of the rolled piece before and after rolling, which is convenient for directly adjusting the roll gap value in production operation.
2) Machining rate: used to record the approximate deformation degree.
3) Paving: used to indicate the absolute added value of paving on the production site.
The most basic parameters involved in the production site are the above, and others, such as elongation and elongation coefficient, are just for theoretical analysis. Therefore, it is most commonly used to calculate the length and machining rate after rolling according to the volume invariable condition of metal plastic deformation.
3. Establishment of rolling process: The rolling process always goes through four stages, namely, biting stage, traction stage, stable rolling stage and rolling end stage.
1) bite stage: the rolled piece begins to contact the rotating roller, and the roller begins to act on the rolled piece and drag it into the gap between the rollers, thus establishing the rolling process.
2) Pull-in stage: once the rolled piece is bitten by the rotating roll, the drag force of the roll on the rolled piece increases, and the rolled piece gradually fills the roll gap until the front end of the rolled piece reaches the position where the two rolls are connected.
3) Stable rolling stage: after the front end of the rolled piece leaves the roll gap, it continues to pass through the roll gap continuously and stably under the action of the friction force of the rotating roll, resulting in the required deformation, compression in the thickness direction and longitudinal extension.
4) Final rolling stage: entering the deformation zone between the roll gaps from the rear end of the rolled piece until the rolled piece and the roll are completely out of contact.
4. In the actual rolling production, the rolled piece cannot be successfully bitten by the roller, which leads to the stop of the rolling process. The unreasonable biting angle leads to uneven plastic deformation of the plate, which not only reduces the production efficiency, but also causes product quality problems, because the biting rolling process is an unstable process, and the geometric parameters and kinematic parameters of the deformation zone change during biting. Therefore, the biting angle, that is, the central angle of the contact part between the roll and the workpiece, is an extremely important factor in the rolling process. The reasonable biting angle should be between 15 and 20 degrees, and the biting angle increases parabolically with the decrease of roller radius.
5. Stable rolling is the main stage of rolling process, but biting process is the premise of establishing rolling process.
At the moment of biting, the rolled piece is acted by the positive pressure n and tangential friction t of the roll. According to the law of friction, N and T can be decomposed into two forces in the same direction, i.e. vertical direction and horizontal direction, so that the components of N and T are superimposed in two directions, and the rolled piece is compressed in the vertical direction after superposition, resulting in plastic deformation, while the two forces in the horizontal direction are in opposite directions, that is, if you want to bite successfully.
6. Stabilize the biting conditions during rolling.
After the workpiece bites, the rolling enters the pull-in stage. As the workpiece fills the gap, the contact surface between the workpiece and the roller increases, so the position of the stress point of the roller on the workpiece also moves in the turn-out direction, which changes the state of force balance between the rollers. Through formula calculation, α7 is obtained. Any factor that reduces the bite angle of the roll and increases the friction coefficient of the roll surface is beneficial to strengthen the bite and establish a stable rolling process. The usual measures are as follows:
1) Measures to reduce the biting angle and improve the biting. Using large diameter rollers can reduce the contact angle and increase the reduction;
Reducing the reduction can reduce the bite angle, but reducing the reduction will increase the number of rolling passes;
The front end of the rolled piece is made into wedge or arc to reduce the biting angle and can be rolled with large reduction;
Apply horizontal thrust along the rolling direction for forced biting, such as conveying the rolled piece by inertia force of roller table, and apply horizontal thrust for forced biting;
When biting, the roll gap is increased to facilitate biting, and when rolling, the loaded reduction is implemented to increase the deformation during stable rolling.
2) increase the friction coefficient of the roller surface, improve the biting measures, and increase the friction of the roller surface without lubrication when biting;
Low-speed bite rolling can also increase the friction force during bite, improve the bite condition and improve the rolling production efficiency;
According to the relationship between metal friction and temperature, properly changing rolling temperature can increase friction. For most metals, increasing the rolling temperature will increase the friction force because of the existence of oxide scale on the surface of the rolled piece.
8. Metal flow and deformation during rolling
The area where metal plastically deforms between two rolls during rolling is called rolling deformation area, which is surrounded by the contact arc between the rolled piece and the roll, the vertical section where the rolled piece enters the roll and the vertical section where the rolled piece exits. This area mainly involves the length of the deformation zone, which directly affects the metal flow during rolling. Because the main deformation occurs in this area, this area involves more data and more deformation.
9. Two other important terms involved here are sliding forward and sliding backward, which are closely related to our production.
When the metal is rolled from the thickness h before rolling to the thickness h after rolling, the thickness of the rolled piece entering the deformation zone gradually becomes thinner. According to the condition that the plastic deformation volume is constant, the secondary flow passing through any cross section in the deformation zone must be equal. As the rolled piece becomes thinner, the horizontal speed of the rolled piece moves higher and higher from the inlet speed to the outlet speed. As a result, the forward speed of the rolled piece in the forward sliding zone is higher than the linear speed of the roll surface, that is, the rolled piece slides forward relative to the roll surface, and vice versa.
In fact, the forward slip value during rolling is generally 2%- 10%, and the forward slip is of great significance to the coiling of strip and the tension control before and after continuous rolling. In our on-site production, we can clearly see the traces left by the forward sliding, especially the imprint of the whole material head shape in the paving pass.
10, high cross-section flow and deformation of rolled piece.
A large number of experimental studies and theoretical analysis show that the flow and deformation in the rolling deformation zone are uneven, which is mainly caused by the influence of contact friction. The greater the friction, the more uneven the horizontal speed. On the same section, the greater the velocity difference between two adjacent layers with different heights, the greater the deformation. In addition, the shape coefficient of deformation zone has great influence on the deformation distribution in the height direction of rolling section. When the rolled piece is thin, the compression deformation will go deep into the center of the rolled piece, resulting in the phenomenon that the deformation of the central layer is greater than that of the surface layer. When the rolled piece is thick, with the reduction of the shape coefficient of the deformation zone, the influence of the outer end on the deformation process becomes prominent, and the compression deformation is difficult to penetrate into the center of the rolled piece, and plastic deformation occurs only in the area near the surface layer, resulting in the phenomenon that the deformation of the surface layer is greater than the center. When rolling a thick rolled piece, due to the increase of contact friction, some alloys in the first few passes of hot rolling have small deformation and large friction, which are easy to stick to the roll, and even lead to the opening of the head of the rolled piece and even roll-up.
1 1. In addition to high compression and longitudinal extension, there is also transverse deformation caused by transverse flow during rolling, which is called spreading. According to the law of metal flowing along the direction of minimum resistance, the metal flowing along the horizontal section can be divided into four areas because of the different effects of friction resistance. As shown in the figure, the deformation zone can be divided into two parts: the extension zone and the wide exhibition zone. In the region and region, the lateral resistance is greater than the longitudinal resistance, and the metal particles flow almost all the way along the longitudinal direction, obtaining extended deformation. In the region and region, the lateral resistance is much smaller than the longitudinal resistance, and the metal particles flow in the lateral direction to produce a wider spread. It can be seen that the spreading mainly occurs in rolling. As the friction resistance increases from the edge to the center of the rolled piece, the metal particles closer to the edge tend to flow laterally, while the metal particles at the center tend to flow longitudinally, that is, the longitudinal flow speed of the metal particles at the center is faster than that at the edge, which is the reason why the head of the rolled piece is fan-shaped and the tail is fishtail-shaped. If the additional tensile stress caused by the speed difference between the center and the edge exceeds the metal strength limit, edge cracks will occur. Communication is actually a very complicated process. At present, we don't have a clear method to calculate propagation. The calculation of spread is mostly inferred from measured data or from actual operation experience, so there is a lot of research space in this field.