1.1 Device Features
The twenty-high Sendzimir mill is currently the largest rolling mill for ultra-thin strips, and is suitable for rolling alloy strips with high deformation resistance. In the twenty-high Sendzimir mill, a pair of small-diameter work rolls are supported by 18 backup rolls arranged in three rows, which have good rigidity in all directions, and can withstand large rolling force and horizontal strip tension. Low energy consumption, high precision of finished products, and thin thickness have the following advantages:
(1) The diameter of the work roll is small, the minimum rollable thickness is reduced, and the rolling pressure is low. In the twenty-high Sendzimir mill, the diameter ratio of the work roll and the backup roll can reach 1:10, while the four-high cold rolling mill is generally 1:3~1:4, so the length of the deformation zone can be greatly reduced. The unit pressure increases under the rolling force, and the rolling can be carried out with a large reduction.
(2) The arrangement and structural characteristics of the roll system can ensure that the small-diameter work rolls have high rigidity on the vertical and horizontal planes along the length of the roll body.
(3) Due to the small diameter of the work rolls, the elastic flattening of the work rolls is very small, and the hard-to-deform metals and alloys can be rolled with fewer passes without intermediate annealing, and very thin strips can be obtained.
(4) Due to the roll shape control device and the strip edge shape control device, the roll shape of the backup roll and the work roll can be controlled during the rolling process. When the width of silicon steel rolled in the United States is 120mm, the thinnest can reach 0.002mm; when the width of stainless steel rolled in Japan is 200mm, the thinnest can reach 0.01mm, and the rolled steel strip can be guaranteed to have high thickness accuracy, flatness and surface quality .
(5) The backing bearing support roller and the overall frame with multi-pivot beams ensure that the working machine base has great rigidity, so that the steel strip can reach a narrow thickness tolerance range in the width and length directions.
(6) Equipped with a computer control system, the rolling process has a high degree of automation, high control precision and fast response speed, and is suitable for rolling high-strength metals and alloys and other materials.
1.2 Factors affecting cold rolling
(1) Friction coefficient (rolling lubricant): Rolling lubricant must also be used for cold rolling of stainless steel, the purpose of which is to reduce the friction on the contact surface of the roll and the steel plate, reduce the rolling pressure and required power, and improve the surface condition And improve the cooling effect of the roll. The lubricants used in cold rolling include mineral oil, palm oil and emulsion. The quality should meet the standard requirements. It should be filtered before use.
The friction coefficient between the roll and the steel plate varies with the type of lubricating oil and the lubrication method, and is also related to the surface state of the roll and steel plate and the conditions (temperature, speed, reduction ratio) during rolling, and is usually between 0.04 and 0.15.
(2) 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. Cold rolling of stainless steel requires a large rolling tension. The tension of the reversible chiller comes from the coilers before and after the rolling mill. Different rolling mills, different steel grades and different thicknesses, set different tensions (of course it should be below the yield point). The rolling tension (unit tension) of the Sendzimir mill is: 390 to 490 MPa for Ni series steel; 295 to 390 MPa for Cr series steel. Generally, the front tension is slightly larger than the back tension, but when producing strips with a thickness of less than 0.5mm, the back tension can be selected to be equal to or slightly larger than the front tension.
(3) Rolling speed: When rolling hard and thin materials, the rolling speed also affects the rolling pressure and the thickness of the steel plate. Multi-high rolling mills are rolled at high speed, for example, the 8-high rolling mill has a maximum passing speed of 300m/min; and the 20-high Sendzimir rolling speed can even reach 800m/min.
(4) Rolls: Rolls used in cold rolling mills must have high hardness, strength and high wear resistance, as well as certain toughness. The materials of the rolls used in the multi-roll cold rolling mill are high-chromium steel and high-speed steel. Different parts of the roller should choose different composition of steel. In order to ensure the surface quality of cold-rolled products, there are strict requirements on the surface of the work roll, not only a certain degree of smoothness, but also no visible defects (cracks, indentations, indentations, grinding lines, spiral patterns, etc.).
Rolls are carefully ground and inspected before use. During the rolling process, the surface of the strip should be checked frequently, and the rolls should be changed immediately if any problems are found. Therefore, when rolling stainless steel, the work rolls are changed very frequently. Generally, each roll needs to be changed at least twice and sometimes even several times. Therefore, a special grinding room should be set up. The lathe for grinding work rolls must have high precision, and the grinding precision is required to reach 5μm (0.005mm).
2 Material properties of incoming coils
At present, there are four main types of stainless steel, namely ferritic stainless steel, martensitic stainless steel, austenitic stainless steel and duplex stainless steel. Solid metals and alloys are all crystals, that is, the atoms in their interiors are arranged according to certain rules, and there are generally three arrangements: body-centered cubic lattice structure, face-centered cubic lattice structure and close-packed hexagonal lattice structure.
Metals are composed of polycrystals, and their polycrystalline structures are formed during the crystallization of the metal. The iron constituting the iron-carbon alloy has two lattice structures: α-iron with a body-centered cubic lattice structure below 910 °C, and γ-iron with a face-centered cubic lattice structure above 910 °C. If carbon atoms are squeezed into the lattice of iron without destroying the lattice structure of iron, such a substance is called a solid solution.
The solid solution formed by dissolving carbon into α-iron is called ferrite, and its ability to dissolve carbon is extremely low, and the maximum solubility does not exceed 0.02%. The solid solution formed by dissolving carbon into γ-iron is called austenite, and its ability to dissolve carbon is high, up to 2%. Austenite is the high temperature phase of iron-carbon alloys.
The austenite formed by steel at high temperatures becomes unstable supercooled austenite when it is supercooled below 727 °C. If it is supercooled to below 230 ℃ at a great cooling rate, the carbon atoms in the austenite have no possibility of diffusion, and the austenite will directly transform into a carbon-containing supersaturated α solid solution, called martensite. Due to the supersaturation of carbon content, the strength and hardness of martensite are increased, the plasticity is decreased, and the brittleness is increased.
3 The main surface quality requirements of the products in the cold rolling process of the twenty-high mill
The rolled product should reach the target thickness of the cold-rolled strip required by the customer’s order, and ensure that it is within the effective tolerance range of the order; improve the metallographic structure of the metal through a certain compression ratio; the product is required to meet certain surface requirements (for example BA bright surface, Super8K mirror surface, 2B surface, 2BB bright surface, No.3/No.4/HL, etc. polished surface, textured surface, patterned surface, etc.).
4 Main defect categories and preventive control measures in cold rolling process of twenty-high mill
There are two reasons for the formation of surface defects: one is the defects brought by the raw materials, exposed during the cold rolling process, and continue to remain or remain; the other is the defects generated during the cold rolling process.
4.1 Defects caused by raw materials
Stainless steel contains high alloying elements, has more developed columnar crystals and has a second phase structure at high temperature, resulting in low thermoplasticity. If surface cracks occur during hot working, even if they are removed by grinding, they tend to remain on the surface of the finished product due to residual defects. If the surface defects of the stainless steel billet are not cleaned thoroughly, it will affect the surface quality of the finished product. During the hot rolling process, due to the insufficient hardness, cooling strength and uniformity of the rolls, defects such as roll marks and grids on the surface of the hot rolled steel strip are caused. These defects cannot be removed in the cold rolling process, and are periodically distributed on the entire surface of the steel strip, which has a great impact on the yield. Surface grinding of hot-rolled steel strips before cold-rolling is the ultimate means to control the surface quality of steel strips before cold-rolling.
4.2 Defects in the cold rolling process
There are many cold rolling process units, and each unit and individual equipment may cause surface defects, which are often continuous or periodic. The following focuses on the description of several main types of cold rolling defects that are likely to occur in the twenty-high mill production line in the cold rolling process and their effective prevention and control measures.
(1) Cold rolling scratches:
This type of defect mainly comes from the head and tail of each coil of strip steel, which is affected by the coiling tension of the reel shaft during rolling, especially for thick materials with a bright surface of BA above 1.2mm. Especially obvious. In view of such defects, attention should be paid to strengthening the monitoring of the coiling tension at the head and tail of the strip during rolling. When rolling thick materials, the stability of tension control must be ensured when the coiling shaft is stretched or unstretched.
(2) Cold-rolled pits or imprints:
Under the operating conditions of large rolling force, such defects will occur when the surface of the work roll is fatigued, or there are small foreign particles adhering to the surface of the work roll. First of all, it is necessary to pay attention to ensure that the hardness of the purchased work rolls meets the technological requirements. In the daily production and operation process, the roll grinder room (RGS) must control the grinding amount of the fatigue layer on the roll surface of the work rolls. The cold rolling mill production line (CRM) must strictly control the total number of passes in the rolling with a pair of new work rolls.
(3) Taper roller indentation (tire pattern):
This type of defect is mainly caused by damage to the surface of an intermediate roller. Roll grinding machine room (RGS) must control the amount of grinding of an intermediate roll/roll surface fatigue layer; twenty-high cold rolling mill production line (CRM) must be based on the difference of the stand roll system and the material properties of different steel grades Reasonably distribute the reduction amount of each pass to match the rolling tension, so as to effectively prevent the roll surface from being damaged due to excessive rolling force.
(4) Vibration marks:
The grinding process must be strictly controlled in the roll grinder room (RGS) to avoid the formation of hidden vibration marks on the roll surfaces of the work roll, the first intermediate roll, the second intermediate roll and the backup roll, which will eventually occur on the surface of the strip. Vibration marks at different periods; 20-high cold rolling mill production lines (CRM) must pay attention to prevent slippage in the first few passes of rolling, which may cause continuous vibration on the strip surface that cannot be completely eliminated by the last pass. mark.
(5) Scratches on oil squeeze rollers:
As the twenty-high rolling mill is often in high-speed operation, it is very easy to produce scratches. For the oil squeeze roller system, we must strictly control the regular renewal and maintenance of the oil squeeze roller wiper; and pay attention to the roller diameter matching of the oil squeeze roller and the regrinding quality of the roller surface (such as the roller surface hardness and crown requirements of the wool roller).
(6) Striped or whitish plate surface:
mainly control the reduction amount, cooling oil amount and rolling speed of the first few passes and the last three passes of each roll; use cold rolling rolling oil of suitable composition.
(7) Oil spot defects:
strengthen the control of the oil squeezing effect of the oil squeezing roller wiper, and set the pressure of the wiper reasonably; reasonably select the type of cold-rolled rolling oil, and pay special attention to the annealing and cleaning performance in the characteristics of the rolling oil.
(8) Mandrel coiling crease:
especially for the target thickness below 1.0mm, the appropriate rolling unit tension is adopted according to the material characteristics of the steel coil and the steel sleeve is reasonably selected.
(9) Paper marks or paper spots:
Strictly control the actual reduction amount and rolling speed of the last pass to effectively control the temperature of the strip surface; pay attention to the paper surface roughness of the intermediate pad paper used in the final pass of the rolling mill (Pad paper quality characteristics).
(10) Poor plate shape: Intuitively, the plate shape refers to the warpage of the plate and strip, and its essence is the distribution of residual stress inside the plate and strip. As long as there is residual stress inside the sheet and strip, the sheet shape is poor. If the residual stress is not enough to cause warping of the strip, it is called “potential” poor shape; if the residual stress causes the strip to become unstable and warp, it is called “apparent” bad shape. Common plate defects include edge wave, middle wave, unilateral wave, two-rib wave and compound wave, etc., which are mainly caused by the uneven extension of each part of the strip during the rolling process, resulting in internal stress.
As we all know, the main factors that affect the shape of the plate are as follows: Changes in rolling force, changes in the crown of the incoming sheet, the crown of the original roll, the width of the plate, the tension, the contact state of the roll, the change of the thermal crown of the roll, etc. Wait.
In order to obtain high-quality rolled strips, it is necessary to adjust the roll gap of the rolls at any time to suit the plate crown of the incoming material, and to compensate for the influence of various factors on the roll gap. For strips of different widths, thicknesses, and alloys, there is only one optimum crown for the roll to produce the ideal target shape. Therefore, the essence of flatness control is to control the bearing roll gap. Unlike thickness control, which only needs to control the opening accuracy at the midpoint of the roll gap, flatness control must control the full roll gap shape within the width span of the rolling stock. Based on this, for plates with different specifications and different material properties, the reasonable selection of taper or crown of different roll series in the entire roll system of the twenty-high mill must be considered to ensure the accuracy of shape control during rolling.
5 Conclusion
Due to the complexity of the operating conditions of the entire system of the 20-high rolling mill, the diversity of stainless steel coil material characteristics and the difference in the surface requirements of customer orders, and in the production process of stainless steel sheets, many product defects come from two. In the cold rolling section of the ten-high rolling mill, how to reasonably match and optimize the equipment performance and process parameters of the twenty-high rolling mill to effectively reduce the proportion of major defects in the cold rolling section has always been the main goal of stainless steel sheet production.
