The main factors affecting rolling - 1, cold rolling technology (124)
一、Biting of the rolling stock
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 >, the rolls cannot 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 roll gradually increases, and the contact area between the rolling piece and the rolling piece gradually increases. The action point of the resultant pressure also moves inward gradually, and the relationship between the large bite angle and the friction angle also changes accordingly.
2. Factors affecting the bite of rolled pieces
(1) Influence 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 is different from the size of the contact surface. 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).
(4) 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 biting speed. When the rolling piece is biting, the roll speed is increased to carry out normal rolling.
二、Slide 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:
(V1)×100%
S - forward slip value
V——Circumferential speed of the roll
V1——The speed at which the rolling piece leaves the roll When cold rolling, the forward slip value is generally within the range of 0-6% according to the different conditions such as processing rate, tension and friction coefficient.
1. Among the experimental methods for determining the forward slip value, the notch method is relatively easy to implement, that is, the notch is made on the surface of the roll, and the distance is L. If the indentation distance on the surface of the rolled piece after rolling is L1, the forward slippage 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 adopted, 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 front 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 width, as for 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.
三、Widen
Spread refers to the change in size 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, absolute spread and 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.
四、Friction
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 large bite angle that restricts the smooth rolling process, but the large 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 and data.
Formulation of cold rolling system
According to the performance of cold-rolled products, the formulation of cold-rolling system should consider the following factors:
1. A class of recrystallization diagrams
A type of recrystallization diagram is a graph that reflects the degree of metal deformation and the relationship between the annealing temperature after deformation and the size of the recrystallized grains of the metal. At a certain annealing temperature, when the degree of deformation is very small, the phenomenon of rapid grain growth often occurs. Usually, the range of deformation degree that causes sharp grain growth is called the critical deformation degree. The size of the critical deformation degree varies with metals and alloys, which must be considered during cold rolling, so that the total processing rate should far exceed the critical deformation degree, so that the product has uniform and fine grains after annealing at a certain temperature organization.
2. Properties and orientation of rolled pieces During cold rolling of metals and alloys, the grain shape is elongated, thinned or flattened along the major deformation direction. While the grains are elongated, the inclusions in the metal And the second phase is also elongated or broken, arranged in chains, this organization is called fibrous tissue.
In the process of cold rolling, when a certain degree of deformation is reached, due to the rotation of the lattice orientation in the grain, its specific crystal planes and crystal orientations tend to be aligned in a certain direction, so that the original orientation disorder of the crystal. The grains change in order and have a strict orientation relationship. The structure formed by the metal is called deformation texture.
The generation of fibrous and deformed textures leads to the anisotropy of metals. In the process of cold rolling, the anisotropy tends to be obvious with the increase of the degree of deformation. Usually, the obvious deformation texture occurs when the total processing rate of cold rolling reaches 50%-60%. For example, a material with an obvious deformed texture has been recrystallized and annealed into another texture, that is, a recrystallized texture. This texture is disadvantageous for subsequent forming processes, such as stretch forming, deep drawing and the like.
This material has anisotropy. When it is deep drawn, some directions extend more to form protrusions, and some directions extend less to form depressions or even cracks. The convex or concave parts are distributed symmetrically, forming "ears". The raised portions may be at 0° or 90° and 45° angles to the rolling direction, commonly referred to as 0° or 90° and 45° lugs, respectively.
For industrial pure aluminum sheets with a total cold-rolled processing rate of 90%, the ear-making rate during deep drawing is 5-6%, and 45° ear-making occurs. If intermediate annealing is added, the deep-drawing ear-making rate can be reduced to 1-3%. In order to reduce ear-making (generally less than 3-4%), the total processing rate of cold rolling should not be too large and should be controlled at about 50%. Anisotropy affects the processing performance and product quality of some products. In actual production, the anisotropy of products should be avoided or reduced as much as possible. Homogenization annealing is an effective measure to eliminate anisotropy. In addition, for some special products, Measures such as controlling the processing rate and increasing the iron content can be taken to further reduce the anisotropy.
3. Determination of pass processing rate and total pass processing rate
During the cold rolling process, cold work hardening occurs in the rolling stock, so the plasticity decreases. After the degree of cold deformation reaches 45-55%, the elongation of pure aluminum and almost all aluminum alloys is roughly the same, about 3-6%. But their ability to continue cold rolling is not the same. Therefore, the elongation cannot be used as the basis for formulating the rolling system, and the amount of deformation when cracking occurs can be used as such a standard. The total deformation amount can reach a large value (90-92% for rolling hard aluminum alloys, and more than 95% for rolling soft aluminum alloys), but a small amount of pass deformation will make the surface quality of the rolled piece different from that of rolling. The output quality deteriorates and the productivity of the cold rolling mill is greatly reduced due to the increase in the number of passes. A good edge condition is one of the basic conditions for obtaining high-quality rolled products.
During cold rolling, in order to prevent edge cracking, intermediate annealing is required after a certain amount of total deformation. For some aluminum alloys, a certain total deformation must be achieved, otherwise, it is an annealed product, and the mechanical properties of the product are difficult to guarantee. When the pass processing rate is within the plasticity range of the alloy, and good surface quality and rolling quality can be obtained, it should be as large as possible to reduce the number of rolling passes and improve the production efficiency of the rolling mill, but the size of the pass processing rate should be considered. The equipment conditions are mainly limited by the large rolling force. If the rolling force is too large, it is easy to cause a roll breakage accident, and the processing rate curve of each pass should rise smoothly. When assigning pass rates, consider mill performance, process lubrication and cooling conditions, tension, original roll shape and operating level. When there are special requirements, such as products with strict thickness deviation and high surface quality requirements, a smaller pass processing rate should be selected. When rolling a 0.3-0.5 mm sheet, in order to prevent strip breakage, the pass should be carefully controlled. The processing rate consciously makes the edge of the rolling piece produce small waves at the edge that can be cut off.
4. Rolling speed
With the development of rolling technology, the rolling speed of the rolling mill has been continuously increased. The rolling speed is increased, that is, the auxiliary time is shortened, resulting in high production efficiency.
When using high-speed rolling, a series of problems must be solved, mainly including:
1. Must have a controllable and effective cooling system. In the process of high-speed rolling, it can effectively reduce the temperature of the roll and the rolled piece, and avoid the occurrence of aluminum sticking due to the high temperature of the roll.
2. The various properties of the process lubricant should be able to meet the requirements of high-speed rolling. For example, viscosity-temperature characteristics, lubricating properties, etc. should meet the requirements of high-speed rolling.
3. High-speed rolling requires a corresponding automatic control system. For example, automatic protection devices for fast and accurate parking and belt breakage, etc.
4. It can measure and control the thickness accurately and timely. As described in the relationship between the friction coefficient and the rolling speed, as the rolling speed increases, the friction coefficient decreases, which is reflected in the strip thickness, and when the rolling speed increases, the strip thickness decreases. Therefore, when the rolling mill speed up, slow down or other speed changes, it should be able to automatically adjust the roll gap to make the longitudinal thickness of the strip uniform.
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