Stainless steel strip cold rolling basic knowledge
Stainless steel sheet is usually produced by cold rolling method, cold rolled stainless steel is mainly produced by rolling method, and only small batches of stainless steel plates with individual steel numbers are produced individually. Sheet rolling is most suitable for the production of large size thicker steel plates and trial production of new steel grades.
The production of cold-rolled stainless steel strip is not a simple "cold rolling" process, the typical production process of cold-rolled stainless steel strip also includes: heat treatment of hot-rolled steel strip, pickling, grinding and so on; Smooth; Cross cut into steel plate or longitudinally cut into steel strip; Categorize; Clean and pack.
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1 Cold-rolled stainless steel strip production process characteristics
Due to the characteristics of stainless steel and the special requirements for product quality, the stainless steel cold rolling production process has the following characteristics:
(1) Stainless steel is a kind of high alloy steel, rolling deformation resistance is large. In order to carry out high efficiency and high precision rolling, the rigid rolling mill should be used, and the multi-roll cold rolling mill is generally used.
(2) Multi-roll cold rolling machine generally adopts single-machine reversible rolling method. The head and tail parts wound on the coiler during rolling cannot be rolled and are cut off to become waste products. In order to improve the yield of the product, a section of lead belt must be welded at both ends of the stainless steel belt before rolling, and the lead belt part is not rolled; In addition, if the quality of the hot rolled coil is too small, in order to improve the rolling efficiency and yield; Small steel coils need to be welded into large steel coils in advance; On continuous annealing and pickling units, because of the continuous operation, the head and end connection of the steel strip also need to be welded. Therefore, welding is an indispensable link in the production of stainless steel belts. The welding of stainless steel is much more difficult than ordinary steel welding, especially when the weld needs to be rolled, the requirements for welding quality are more stringent, therefore, the special welding process is also a feature of the cold rolling production of stainless steel strip.
(3) Before the cold rolling of stainless steel strip, the raw material (hot rolled coil) should be annealed, the middle annealing in the cold rolling process, and the final product should be annealed, so annealing is an important link in the production. There are many kinds of stainless steel, different properties of various steels, the purpose, methods and requirements of heat treatment are not the same, and there is a unique process system.
(4) The surface quality requirements of cold-rolled stainless steel strip are very strict, not only do not allow the residual metallurgical defects and processing defects caused by the previous process, but also do not allow the obvious defects caused by the cold rolling process. Therefore, a series of elimination and protection measures have been taken in the process of cold rolling. For example, in order to eliminate the hot-rolled oxide sheet, the hot-rolled steel strip should be treated with shot blasting and pickling. In order to eliminate the defects caused by billet and cold rolling; For the defects caused by heat treatment, the steel strip is often trimmed on the grinding unit; In order to ensure the surface quality after cold rolling, the grinding of the roll has very strict requirements, in order to prevent the steel strip from being scratched during the production process, the steel coil of each unit is required to be tightly rolled and fully rolled, and many units before and after cold rolling should be padded with protective paper between the layers of the steel coil; In addition, some special protective measures have been taken on operations and equipment that are prone to scratches; In order to get a good, uniform surface gloss, some finished products after annealing but also pickling; Bright steel plates with special requirements should also be annealed in a protective atmosphere; Some products surface coating and so on. In short, the production of cold-rolled stainless steel strip is a fine and detailed process, which is incomparable to other steels.
(5) There are some special requirements for the finishing of stainless steel belts. For example, the flattening process not only needs to improve the shape of the plate, but also needs to use a roll with very low roughness (except pockmarked plates) to produce bright 2B steel plates, and for ferritic stainless steel, the formability of the steel plate should be improved by controlling the flattening amount. Stainless steel because the steel is hard; At the same time, the flatness requirements are strict, and must be straightened by a strong straightener, and different thickness products often need to use different straightening methods; Stainless steel cold rolled products are usually delivered in two ways: roll and cut plate, so modern stainless steel cold rolling plants are equipped with longitudinal cutting units and cross-cutting units.
2 Cold rolled stainless steel strip production technology
2.1 Types of cold rolling machines
Rolling stainless steel this kind of high alloy high hardness and cold rolling process hardening tendency of the material, and requires the rolling to achieve high efficiency, high precision, must use rigid rolling mill.
Initially, stainless steel cold rolling mostly adopts four-roll reversible rolling machine, although the four-roll rolling mill work roll, support roll diameter is large, the arch is also large, but the rigidity of the rolling mill is not high. In view of this situation, MKW partial eight roll mill, twelve roll mill, twenty roll mill and other multi-roll mills have been gradually developed.
The multi-high mill has the following advantages over the four-high mill:
(1) The multi-roll mill (12-roll, 20-roll mill) of the tower support roll system, the whole roll body of the work roll is supported on the mill yard by the saddle of the support roll, and the bending deformation in the width direction of the roll is small, so that the work roll with small diameter can be used;
(2) Due to the increased stiffness of the mill and the use of high hardness rolls, while the work roll can be quickly replaced, it is possible to produce high-precision, high-quality surface steel strip;
(3) It provides the possibility for the production of wide thin steel strips with a thickness of 0.5-0.05mm;
(4) In the multi-roll mill, the implementation of intensive pressing, so that the mill passes are reduced, and it is possible to reduce the rolling distance, so as to improve production efficiency and reduce costs;
(5) Small volume and light weight of the rolling mill; The equipment cost is relatively reduced.
2.2 Factors affecting cold rolling
A friction coefficient (rolling lubricant)
Stainless steel cold rolling mills must use rolling lubricants, especially when rolling on multi-roll mills (12-roll, 20-roll mills). Its purpose has two: first, the process lubrication, reduce the friction coefficient of the roll and steel strip contact surface, reduce the rolling pressure and the required transmission power, improve the surface state of the product; The second is to cool the roll and the steel strip, to remove the deformation generated in the rolling process, so that the mill is in normal working condition.
The lubricants used in multi-roll rolling mill are mineral oil and emulsion, etc. The friction coefficient of roll and steel strip varies according to the type of lubricating oil and lubrication method, and is related to the surface state of roll and steel strip, rolling temperature, rolling speed, reduction rate and other conditions, usually between 0.04 and 0.15.
B Front and rear tension
The tension during cold rolling has a great influence on the rolling pressure, and is also an important factor affecting the shape and thickness of the plate. Multi-roll mill is the use of high tension rolling, stainless steel strip cold rolling requires greater tension. In a single-stand reversing mill, the tension is generated between the mill and the winder. The coiler that forms the front tension is in an electric working state, while the coiler that forms the back tension is in a power generating working state. In the rolling process, constant tension control is generally used, to maintain the tension does not change, because the diameter of the former winder coil is getting larger and larger, the winder should gradually slow down; The coil diameter of the coiler is getting smaller and smaller, so it should be gradually accelerated to keep the corresponding line speed unchanged.
Different rolling mills, different materials and different product specifications, set different front and rear tension. The maximum unit tension value is generally in the range of (0.3-0.5) values. The tension of Ni-Cr series stainless steel strip is 390~490MPa by using 20-high Senjimir mill. The tension of rolled chrome stainless steel strip is 295-390mpa. Generally, in the first rolling process, due to the small winding tension of the pickling unit, etc., large tension can not be used to avoid mismoving between the winding layers and chafing the surface. High tension rolling can be applied in subsequent passes, depending on the rolling variety and specifications, or the front tension is greater than the back tension, or the back tension is greater than the front tension. When the ferritic stainless steel strip is rolled and annealed by the cover furnace after rolling, the unit tension of the finished pass should be appropriately reduced to prevent bonding during annealing. When the weld passes the roll, the tension should also be appropriately reduced.
C Rolling speed
The change of rolling speed will affect the friction coefficient between the roll and the steel strip, thus affecting the rolling pressure and the thickness of the steel strip. Multi-roll mill rolling speed is high, modern 20-roll mill rolling stainless steel strip rolling speed can reach 800m/min(bandwidth is not less than 1110mm) and 1000m/min(bandwidth is 1320mm).
The high rolling speed requires that the hot rolled strip has completely good performance, because some defects on the material, the high rolling speed will have more impact than the lower rolling speed, resulting in greater damage to the rolling equipment and products. Therefore, a lower rolling speed is applied when the first roll is arranged in order to make an accurate inspection of the rolled steel strip and record the location of the defects affecting the high speed rolling. Starting from the second pass, the rolling speed is increased and the defective parts are slowed down under the condition of utilizing the mill motor capacity and strip tension.
D roll
The roll used in the cold rolling mill must have high hardness, strength and wear resistance, and must also have a certain toughness. The roller material used in multi-roll cold rolling mill is high chromium steel and high speed steel. Different parts of the roller should choose different components of steel. In order to ensure the surface quality of cold rolled products, there are strict requirements for the surface of the work roll, not only to have a certain roughness, but also not to have any visible defects (cracks, pits, indentations, grinding lines, spiral lines, etc.). The rolls are carefully ground and inspected before use. During the rolling process, it is necessary to check frequently and change the roll immediately if any problem is found. So when rolling stainless steel strips
Roll change is very frequent, generally each roll to change a roll, sometimes to change several times.
2.3 Down the system
The reduction system includes the regulation of rolling pass, reduction amount and rolling process. The push-down system should be determined according to the characteristics of the mill (mill type, main motor power, rolling pressure, roll hardness, process lubrication, coiling motor power and tension, etc.), and vary with the variety, size and heat treatment quality of the rolled product. Reasonable reduction system is of great significance for improving rolling efficiency, product quality and reducing consumption. In fact, when making a downsizing plan; The push down system is determined according to the pressure of the metal on the roll and the load of the main motor. Usually in the first two passes, the reduction is limited by the load of the main motor. In cold rolling, a large reduction is often prescribed in the first pass, and the reduction is gradually reduced in subsequent passes in order to maintain the same metal pressure on the roll in each pass.
In some cases, roll passes are significantly increased in order to produce work hardened strips or to produce strips with minimal warpage and uniform thickness. When rolling, the unit pressure and total pressure of the metal on the roll shall not exceed the design standard. For the four-roll mill unit pressure is not more than 1700MPa; For multi-roll mill unit pressure in the range of 2000 ~ 2500MPa. The reduction amount of each pass and the total reduction amount of a roll also depend on the mechanical properties of the rolled steel strip and their changes during the rolling process. If quenched, the =320MPa, =50% 1Crl8Ni9Ti stainless steel strip is rolled from 3mm thick to 1.5mm thick on the 1680mm four-roll reversible cold rolling machine with 7 passes. After quenching treatment, =600MPa, = 26% of 1Crl8Ni9Ti stainless steel strip under the same conditions, the thickness of 3mm rolled to 1.5mm to use 9-11 rolling times to complete. Process lubrication and roll surface roughness have great influence on the reduction system. For example, the 1Crl8Ni9Ti stainless steel strip is rolled from 3mm to 1.5mm in 11 passes on a 1680mm four-roll reversing mill, using No. 20 oil as a process lubricant and rolling with a roll with surface roughness Ra= 0.8 or Ra= 0.4. However, by using a roll with a surface roughness of Ra= 0.1 to roll the same specification product, the rolling passes may be reduced to 9 or even 7.
With effective process lubrication, it is possible to increase the total reduction rate of a roll to 80% or more. According to the traditional process, when using No. 20 oil, the billet with a thickness of 3mm is rolled to the finished product with a thickness of 0.7mm with two rolling steps (intermediate heat treatment when the strip thickness is 1.5mm), the total pass is 16, and the total reduction rate between the two heat treatments is 50% and 53.3%. When a bright oil called 28 is used, under the condition that the passes can be reduced to 11 passes, the total reduction rate of 76.7% is rolled in one roll. When using polymer cottonseed oil, it can be used in 13 passes to 81. . The total reduction rate of 6% can roll the large roll billet with a thickness of 3.8mm into a finished product with a thickness of 0.7mm. With such a reduction amount, not only can the austenitic stainless steel with good plasticity, but also the austenite-ferrite and austenite-martensitic stainless steel which is difficult to deform. The forced pressing system will increase the temperature of the steel strip and improve the maggot plasticity of the steel strip. Under this condition, although the total reduction is quite large, there will be no cracking of the steel strip during rolling.
By strengthening the push down in the multi-roll mill, the rolling passes and the intermediate annealing times can be reduced. A: No
Rust steel four-roll mill from 3.0mm thick billet rolling 0.4-0.5mm thick finished products, need to carry out an intermediate heat treatment, and rolling with multi-roll mill, but do not need intermediate heat treatment, without intermediate heat treatment and the use of large reduction can also produce high-quality surface stainless steel strip. Under the condition that the total reduction rate is 97%, the high strength and high stiffness of the multi-roll mill can keep the relative reduction amount of each pass unchanged, so that the power of the main motor can be used more effectively than other types of mills.
2.4 Shape control
The purpose of shape control is to roll out strips with uniform transverse thickness and straight shape. The transverse thickness difference of steel strip and the flatness of steel strip are two different physical concepts in the shape control. The transverse thickness difference is an index indicating the uneven thickness distribution on the cross section of the steel strip. Flatness is an index that indicates that the steel strip loses its flat appearance and has shape defects such as wave shape and warping, which is directly caused by the uneven deformation of the rolled steel strip in the width direction. There is no necessary connection between the two, and the transverse thickness difference of the rolled steel strip is very small, but its flatness may be poor due to uneven deformation. But the two also have a very close relationship, they both depend on the shape of the work roll curve during rolling.
Therefore, the shape control is to adjust the shape of the load roll gap under the condition of good shape of the incoming plate, so as to keep it consistent with the cross-section shape of the incoming steel strip, so as to reduce the transverse uneven deformation and make the rolled plate straight. Adjusting the shape of the loaded roll gap is easy for a variety of 20-high mills. Due to the structural characteristics of 20-high rolling mill, there are many kinds of shape adjusting means, generally there are radial roll shape adjusting mechanism and axial roll shape adjusting mechanism; Split mill has roll tilt adjustment function; Part of the direct press of the twenty-high mill also has a hydraulic roll bending mechanism, etc. For the four-high mill, the most basic method of shape control is to determine the appropriate roll crown, that is, to consider the width of the steel strip in the direction of the pressure distribution characteristics, the roll into the middle part of the slightly convex shape. The modern four-high mill also uses other plate shape adjustment mechanisms, such as roll tilt, hydraulic roll bending, roll axis movement of CVC mill, emulsion flow level control.
In addition, the shape of the plate is also affected by some other operating factors, such as the appropriate tension and rolling pressure. The general shape of the plate is reflected by the size of the tension along the width of the steel strip during the rolling process, which needs to rely on the operator's experience to judge and adjust. In recent years, the on-line measuring device and various control technologies have been more and more widely used in production, especially in the wide strip mill, and will become an important development direction of cold rolling technology in the future.
2.5 Thickness Control
Cold rolled stainless steel strip should be uniform in thickness
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