The production process of hot rolling

Introduction of rolling mill

The process of producing hot rolled strip with thickness of 1.2 ~ 8mm on hot strip mill. Strip width below 600mm is called narrow strip steel; Over 600mm is called wide strip steel. The first hot strip rolling mill was put into operation in the United States in 1905, producing 200mm wide strips. The technical and economic index of hot strip mill is superior and develops quickly. In industrialized countries, the output of hot rolled wide strip steel accounted for about 25% of the total steel output before 1950, and reached about 50% in the 1970s. The raw material of hot rolled strip is continuous casting slab or preliminary rolling slab with thickness of 130 ~ 300mm. After the slab is heated in the furnace, it is sent to the mill for rolling into strips of 1.00 ~ 25.4mm thick, and then rolled into steel coils. Rolled steel types are ordinary carbon steel, low alloy steel, stainless steel and silicon steel. It is mainly used as raw material for cold rolled strip steel, welded pipe, cold bending and welded section steel. Or for the production of various structural parts, containers, etc.

Rolling mill composition

Hot strip mill consists of roughing mill and finishing mill.

Roughing mill components semi-continuous, 3/4 continuous and fully continuous three types:

(1) The semi-continuous type has a scale-breaking (remove the oxidation sheet) frame and a reversible frame with a vertical roll;

② In the 3/4 continuous type, in addition to the above frame, there are two serial continuous arrangement frames;

The fully continuous type is composed of 6 ~ 7 frames. The finishing mill group is composed of 5 ~ 7 continuously arranged stands and coiler. Hot strip mills are named according to the length of the roll. Those with a roll length of more than 914 mm are called wide strip mills. Finishing mill work roll body length of 1700 mm, called 1700 mm hot strip mill, this mill can produce 1550 mm wide strip coil.

There are four kinds of hot strip rolling according to product width and production process: wide hot strip rolling, wide reversible hot strip rolling, narrow hot strip rolling and hot strip rolling with planet mill.

Principle of hot rolling

1. The theory of rolling: Rolling is a material processing method that drags rolled pieces into rolls with the help of the friction force of rotating rolls, and at the same time relies on the pressure exerted by rolls to compress and deform rolled pieces between two rolls or more than two rolls.

2. rolling parameters: rolling deformation process, thickness direction compression is the dominant deformation. When the thickness of the rolling part is compressed by the roll, the metal will occur along the longitudinal and transverse flow, but the longitudinal extension deformation is always greatly more than the transverse extension, this is because the roller friction on the wide flow obstruction is always greater than the longitudinal, that is to say, relative to the longitudinal, the transverse width is always relatively small, the deformation index of rolling is mainly:

1) Absolute reduction: represents the absolute change of the thickness of the rolled parts before and after rolling, which is convenient to directly adjust the value of the roll gap in the production operation.

2) Processing rate: used to record approximate deformation degree.

3) Wide exhibition: the absolute value added of wide exhibition is used in the production site.

The most basic parameters involved in the production site are the above several, others such as elongation, elongation coefficient, just for theoretical analysis. Therefore, the most used in the field is to calculate the length and processing rate after rolling according to the constant condition of metal plastic deformation volume.

3. the establishment of rolling process: the rolling process goes through a total of 4 stages, respectively, the biting stage, the pulling stage, the stable rolling stage and the end of the rolling stage.

1) Bite stage: the rolling part begins to contact the rotating roll, and the roll begins to exert an effect on the rolling part, pulling it into the roll gap in order to establish 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 seam until the front end of the rolled piece reaches the position of the core line of the two rolls.

3) Stable rolling stage: After the front end of the rolling piece comes out from the roll seam, it continues to rely on the action of the rotating roll friction on the rolling piece, and continuously and stably passes through the roll seam to produce the required deformation, thickness compression and longitudinal extension.

4) The end of rolling stage: from the end of the rolling part into the deformation zone between the cracks until the rolling part and the roll completely out of contact.

4. the actual rolling production will appear rolling parts can not be smoothly bitten into the roll, causing the rolling process to stop, and the bite Angle is unreasonable cause uneven plastic deformation of the plate, not only reduces the production efficiency, and the product is easy to have quality problems, this is because the process of biting and rolling is an unstable process, when the bite, the geometric parameters of the deformation area, Kinematic parameters are variable, so the bite Angle, that is, the center Angle of the contact part between the roll and the rolling piece, is an extremely important factor in the rolling process. The reasonable bite Angle should be between 15 degrees and 20 degrees, and when the radius of the roll is the same, the bite Angle increases with the amount of pressure parabola.

5. stable rolling is the main stage of rolling process, but the biting process is a prerequisite for the establishment of rolling process.

At the moment when the rolling piece is bitten, the rolling piece is affected by the positive pressure N of the roll and the tangential friction force T. According to the law of friction, N and T can be decomposed into two forces in the same direction, namely in the vertical direction and in the horizontal direction. In this way, there is the superposition of the components of N and T in the two directions. However, the two horizontal forces are in opposite directions, that is, if the bite is to be successful, the horizontal component of T is greater than that of N. Thus, an index, namely the bite Angle, is fixed to determine the bite state.

6. stable rolling bite conditions

The contact surface between the rolled and the roll increases with the filling of the rolled to the gap between the rolls. Therefore, the position of the force point of the roll on the rolled moves to the direction of the roll out, and the force balance between the rolls changes. α&lt is obtained by the formula. 2β is the critical condition for stable rolling.

7. All the factors that reduce the bite Angle of the roll and increase the friction coefficient of the roll face are conducive to strengthening the bite and establishing a stable rolling process. The usual measures are as follows:

1) Reduce roll bite Angle and improve bite measures

The use of large diameter roll, can reduce the contact Angle, and is beneficial to increase the amount of pressure;

Although the bite Angle can be reduced by reducing the amount of pressing, the rolling pass will be increased by decreasing the amount of pressing.

The front end of the rolling part is made into a wedge or arc shape to reduce the bite Angle, which can implement large pressure rolling;

Apply horizontal thrust along the rolling direction for forced biting, such as the inertia force of rolling parts transported on the roller table to apply horizontal thrust for forced biting;

When biting, the roll seam is raised to facilitate biting. When rolling, the deformation of stable rolling is increased by pressing down with load.

2) Increase the friction coefficient of roll surface and improve the biting measures

The roller surface is not lubricated when biting, and the friction of the roller surface is increased.

Low speed bite, tell the rolling, can also increase the biting friction, improve the biting conditions, and improve the rolling production efficiency is also favorable;

According to the relationship between metal friction and temperature, the friction can be increased by changing the rolling temperature appropriately. For most metals, increasing the rolling temperature can increase the friction due to the existence of oxide skin on the rolled surface.

8. Metal flow and deformation during rolling

Rolling metal in the two roll seam plastic deformation of the area called rolling deformation zone, the area by rolling and roll contact arc, rolling into the vertical section and the vertical section of the exit of the area, the area is mainly involved in an important index is the length of the deformation zone, the length directly affects the flow of metal rolling. Because the main deformation occurs in this region, this region involves more data, more deformation.

9. There are two other important terms involved here: front slip and back slip, which are closely related to our production

When the metal is rolled from the thickness of H before rolling to the thickness of h after rolling, the thickness of the rolled piece entering the deformation zone decreases gradually. According to the constant volume condition of plastic deformation, the flow rate per second of any cross section through the deformation zone must be equal. As the rolling piece is thinner and thinner, the horizontal velocity of the rolling piece moving from the speed of the inlet roll to the speed of the outlet roll becomes higher and higher. The result is that the forward speed of the rolled piece in the front sliding zone is higher than the linear speed of the roll surface, that is, the rolled piece is sliding forward relative to the roll face, otherwise, the rolled piece in the back sliding zone is lower than the roll surface speed, and the two speeds are equal only on the neutral plane.

In fact, the front slip value of rolling is generally 2%-10%, the front slip for strip rolling take-up, continuous rolling before and after tension control has an important significance. In our on-site production, we can obviously see the marks left by the front slide, especially the flat pass and the impression of the whole material head shape.

10. High flow and deformation on the rolled section

A large number of experimental studies and theoretical analysis show that the flow and deformation in the rolling deformation zone is uneven, the main reason is caused by the influence of contact friction, the greater the friction, the more uneven the horizontal velocity, which is on the same cross section, the greater the velocity difference between the particles at two levels of different heights, the greater the deformation. In addition, the shape coefficient of the deformation zone has a great influence on the distribution of the deformation in the high direction of the rolling section. When the rolling piece is relatively thin, the compression deformation will penetrate deep into the center of the rolling piece, and the deformation of the center layer is larger than that of the surface layer. When the rolling piece is relatively 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 deep into the center of the rolling piece. Plastic deformation occurs only in the area near the surface, and the surface deformation is larger than the core. When thick rolling parts, because of the increase of contact friction, some alloy hot rolling head several times of deformation is small, coupled with large friction, easy to appear sticky roll, resulting in rolling parts head mouth, serious will also wrap the roll.

11. rolling metal in addition to high compression and longitudinal extension, there is also transverse deformation caused by transverse flow, known as wide spread. According to the law of metal flow along the direction of minimum resistance, the flow of metal along the horizontal section can be divided into four zones due to the different influence of friction resistance, as shown in the figure, the deformation zone can be divided into extension zone and wide extension zone. In zone and zone, the transverse resistance is greater than the longitudinal resistance, and the metal particles almost all flow in the longitudinal direction to obtain the extended deformation. The transverse resistance is much smaller than the longitudinal resistance, and the metal particle flows in the transverse direction to produce a wide spread. It can be seen that the wide spread is mainly generated at the edge of the rolled piece, and the back sliding zone is more than the front sliding zone. Because the friction resistance from the rolling part of the edge to the center is larger and larger, so the metal particle closer to the edge of the transverse flow trend is larger, on the contrary, the metal particle in the center of the longitudinal flow trend is larger and larger, that is, the metal particle in the center of the longitudinal flow faster than the edge, which is why the head of the rolling part is fan-shaped, and the tail is fishtail. If the boundary stress caused by the velocity difference between the center and the edge exceeds the strength limit of the metal, the edge crack will appear. Broadening is actually a very complicated process, and we don't have a clear method to calculate broadening at present. Most of the calculation of broadening is inferred from the measured data, or it is obtained from the actual experience of field operation, so there is a lot of room for research in this aspect.

Determination principle

1. There are many ways of hot rolling, but the most common and the simplest is longitudinal rolling, which is the way of rolling along the length direction of the ingot. In the rolling process, it is mainly the interaction between the roll, the rolled parts and the emulsion, and the vertical roll edge rolling outside the roll slit, and the control of the winding tension.

2. Factors affecting rolling

1) Roll parameters, the main parameters affected by this factor are roll shape and surface roughness, and the formulation of these two parameters can be determined according to the actual experience, according to the rolling mill production process requirements, such as the selection of hot rolling mill surface roughness requirements should not only be conducive to biting, prevent slipping in the rolling process, It is also necessary to prevent the rough roller surface from affecting the surface quality of the product. Surface roughness refers to the micro-set shape unevenness with small spacing and small peaks and valleys on the machined surfaces of parts. Although so defined, it contains a surface state that has been ground by a specific grinding process. For example, the same grinding of a work roll with a roughness of 1.0, the effect of grinding with No. 80 grinding wheel and with No. 150 grinding wheel will be very different, such as different grinding fluid, different grinding technology processing effect will also be different.

2) Hot rolling temperature

The hot rolling temperature includes the opening rolling temperature and the final rolling temperature. The opening rolling temperature is determined mainly according to about 80% of the solid phase line temperature in the alloy phase diagram, and the final rolling temperature is determined according to the plastic diagram of the alloy, which is generally controlled above the recrystallization temperature of the alloy.

3) Rolling speed

In general, in order to improve the production efficiency, ensure a reasonable final rolling temperature, in the scope of equipment allowed as far as possible to use high speed rolling, and in the actual production should be determined according to different rolling stages of different rolling speed, such as the beginning of rolling stage, tiling stage, coiling stage, different stages can be used for rolling at different speeds.

4) Hot rolling down system

The determination of hot rolling down system mainly includes the determination of hot rolling total processing rate and pass processing rate, and the determination principle of hot rolling total processing rate is:

a) Properties of alloy materials, such as pure aluminum, high temperature plastic range is wide, hot rolling brittleness is small, low deformation resistance, so the total processing rate is large, but duralumin, hot rolling temperature range is narrow, hot brittleness tendency is large, its total processing rate is usually smaller than soft aluminum alloy.

b) To meet the requirements of surface quality and performance of final products, such as cold rolled billet supply, the total processing rate of hot rolling should be sufficient for cold deformation, in order to control product performance and obtain good cold rolled surface quality.

c) Mill capacity and equipment conditions, the larger the difference between the maximum working opening degree and the minimum rolling thickness of the mill, the thicker the ingot, the larger the total processing rate of hot rolling, but the ingot thickness is limited by the opening degree of the mill and the length of the roll table.

The principle of determining the process rate of hot rolling:

The high temperature performance of the alloy, biting conditions, product quality requirements and equipment capacity should be considered in the formulation of the pass processing rate. The principle of determining the pass processing rate at different rolling stages is:

a) At the beginning of the rolling stage, the pass processing rate is small, generally 2%-10%, because the first few passes are mainly to change the casting structure to the processing structure, to meet the bite conditions.

b) In the middle rolling stage, with the improvement of metal workability, if the equipment conditions permit, the pass deformation amount should be increased as far as possible, the pass processing rate of duraluminum-alloy can reach more than 45%, and the soft aluminum alloy can reach 50%. The large reduction amount of rolling will produce large deformation heat, supplement the heat loss of strip in the rolling process, which is conducive to the maintenance of normal rolling.

c) In the final rolling stage, the general pass processing rate is reduced. In order to prevent coarse grains from being produced in hot rolled products, the processing rate of the last pass of hot rolling should be greater than the critical deformation amount (15%-20%). The temperature of the last two passes of hot rolling is low, and the deformation resistance is large.

Hot rolling purpose

1. suitable for general structural steel and engineering hot rolled steel plate, steel belt, for welding pipe, cold rolling material, bicycle parts, as well as important welding, riveting, bolted components.

2. for cold rolling, deep drawing products.

3. the production of automobile, tractor, engineering lifting machinery, small light industrial civil machinery stamping and structural parts.

4. Make containers and exposed metal structural parts.

5. welded pipe, steel structure.

6. Pipelines for transporting oil and natural gas.

7. car girder, beam.

8. Special steel belt for automobile wheels.

9. structure with anti-skateboard, stairs, ladder pedals, etc.