1 Introduction to 20-Roll Rolling Mill (CRM)

1.1 Equipment characteristics

The 20-roll Senzimir mill is currently the mill with the largest number of rolls for rolling extremely thin strips and is suitable for rolling alloy strips with high deformation resistance. In the 20-roll Senjimmy mill, a pair of small-diameter work rolls are arranged in three rows by 18 support rolls and supported in sequence. They have good rigidity in all directions, can withstand large rolling forces and horizontal strip tension, have low rolling energy consumption, high finished product precision and thin thickness, and have the following advantages:

(1) The diameter of the work roll is small, the minimum rolling thickness is reduced, and the rolling pressure is relatively low. In a 20-high Sendzimir rolling mill, the diameter ratio of the work roll to the support roll can reach 1:10, while in a four-high cold rolling mill, it is generally 1:3 to 1:4. Therefore, the length of the deformation zone can be significantly reduced. Under a given rolling force, the unit pressure increases, and a large reduction in pressure can be adopted for rolling.

(2) The arrangement form and structural features of the roller system can ensure that the small-diameter working rollers have high rigidity along the length of their roller bodies in both vertical and horizontal planes.

(3) Due to the small diameter of the work rolls, the elastic flattening of the work rolls is very small. Therefore, it is possible to roll difficult-to-deform metals and alloys in fewer passes without intermediate annealing, and very thin strips can be obtained.

(4) Due to the installation of the roll profile control device and the edge plate profile control device for the steel strip, the roll profiles of the support roll and the working 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 rolling stainless steel with a width of 200mm in Japan, the thinnest thickness can reach 0.01mm, and it can ensure that the rolled steel strip has very high thickness accuracy, flatness and surface quality.

(5) The adoption of multi-point beam backing bearings to support the rollers and the integral frame ensures that the working base has great rigidity, thereby enabling the steel strip to achieve a very narrow thickness tolerance range in both width and length directions.

(6) It is equipped with a computer control system, featuring a high degree of automation in the rolling process control, high control accuracy and fast response speed, making it suitable for rolling high-strength metals and alloys and other materials.

1.2 Factors influencing cold rolling

(1) Coefficient of friction (Rolling lubricant) : Rolling lubricants must also be used in stainless steel cold rolling. The purpose is to reduce the friction on the contact surface between the rolls and the steel plate, decrease the rolling pressure and the required power, improve the surface condition and enhance the cooling effect of the rolls.

The lubricants used in cold rolling include mineral oil, palm oil and emulsions, etc. Their quality should comply with the standard requirements. They need to be filtered before use. Most importantly, no residue should be left on the plate surface during subsequent annealing.

The coefficient of friction between the roll and the steel plate varies depending on the type of lubricating oil and the lubrication method, and is also related to the surface condition of the roll and the steel plate as well as the conditions during rolling (temperature, speed, and reduction rate). It is usually between 0.04 and 0.15.

(2) Front and back tension: The tension during cold rolling has a significant impact 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 relatively 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 require different tension Settings (of course, it should be below the yield point). The rolling tension (unit tension) of the Sendzimir rolling mill is: 390 to 490MPa for Ni series steel. The mpa of Cr series steel is 295 to 390MPa. Generally, the front tension is slightly greater than the rear tension. However, when producing strips with a thickness of less than 0.5mm, the rear tension can be selected to be equal to or slightly greater 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 roll at high speeds. For instance, the maximum plate passing speed of an 8-high rolling mill can reach 300m/min. The rolling speed of the 20-roll Sendzimir can even reach 800 meters per minute.

(4) Rolls: The rolls used in cold rolling mills must have high hardness, strength and wear resistance, and also must have a certain degree of toughness. The material of the rolls used in multi-roll cold rolling mills is high-chromium steel and high-speed steel. Rollers at different parts should be made of steel of different compositions. To ensure the surface quality of cold-rolled products, there are strict requirements for the surface of the work rolls. They not only need to have a certain degree of smoothness but also must not have any visible defects to the naked eye (such as cracks, dents, indentations, grinding marks, spiral marks, etc.). All rolls must be carefully ground and inspected before use. During the rolling process, the surface of the steel strip should be frequently inspected. If any problems are found, the rolls should be replaced immediately. Therefore, when rolling stainless steel, the work rolls need to be replaced very frequently. Generally, at least two rolls need to be replaced for each roll rolled, and sometimes even several times. Therefore, a dedicated grinding room should be set up. The lathe for grinding the work roller must have high precision, with a grinding accuracy requirement of 5μm (0.005mm).

2 The material properties of the incoming steel coil at the entrance

At present, there are mainly four types of stainless steel grades, namely ferritic stainless steel, martensitic stainless steel, austenitic stainless steel and duplex stainless steel. Solid metals and alloys are all crystals, meaning that the atoms inside them are arranged according to certain rules. Generally, there are three ways of arrangement: 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 process of metals. The iron that makes up the iron-carbon alloy has two lattice structures: below 910℃, it is α -iron with a body-centered cubic lattice structure, and above 910℃, it is γ -iron with a face-centered cubic lattice structure. If carbon atoms are squeezed into the lattice of iron without destroying the lattice structure that iron possesses, such a substance is called a solid solution. The solid solution formed when carbon dissolves into α -iron is called ferrite. Its carbon solubility is extremely low, with a maximum solubility not exceeding 0.02%. The solid solution formed when carbon dissolves into γ -iron is called austenite, which has a relatively high carbon solubility, up to 2% at most. Austenite is the high-temperature phase of iron-carbon alloys. The austenite formed in steel at high temperatures becomes unstable undercooled austenite when undercooled to below 727 ℃. If the austenite is supercooled to below 230 ℃ at an extremely high cooling rate, at this point, the carbon atoms in the austenite have no possibility of diffusion, and the austenite will directly transform into a carbon-supersaturated α solid solution, known as martensite. Due to the supersaturation of carbon content, the strength and hardness of martensite increase, its plasticity decreases, and its brittleness grows.

3 The main surface quality requirements for the products of the cold rolling process of the 20-high rolling mill

The rolled products should reach the target thickness of cold-rolled strip steel as required by the customer's order and ensure that it is within the effective tolerance range of the order. Improve the metallographic structure of metals through a certain compression ratio; The product is required to meet certain surface requirements (such as: BA bright surface, Super8K mirror surface, 2B surface, 2BB bright surface, No.3/No.4/HL and other polished surfaces, roughened surfaces, patterned surfaces, etc.).

4 The main defect categories of the cold rolling process of the 20-high rolling mill and their preventive and control measures

There are two reasons for the formation of surface defects: one is the defects brought by the raw materials, which are exposed during the cold rolling process and remain or persist. The second is the defects that occur during the cold rolling process.

4.1 Defects caused by raw materials

Stainless steel contains a relatively high amount of alloying elements, has well-developed columnar crystals and a second-phase structure at high temperatures, resulting in low hot plasticity. If surface cracks occur during hot working, even if they are eliminated by grinding, they often remain on the surface of the finished product due to residual defects. If the surface defects of stainless steel billets are not thoroughly cleaned, it will affect the surface quality of the finished product. During the hot rolling process, due to insufficient hardness, cooling intensity and uniformity of the rolls, defects such as roll marks and grids on the surface of the hot-rolled steel strip occur. These defects cannot be removed in the cold rolling process and are periodically distributed throughout the surface of the steel strip, having a significant impact on the yield rate. Surface grinding of hot-rolled steel strips before cold rolling is the ultimate means to control the surface quality of the steel strips in the pre-cold rolling process.

4.2 Defects produced 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 elaborating several major types of cold rolling defects that are prone to occur in the 20-high rolling mill production line during the cold rolling process and their effective preventive and control measures.

(1) Cold rolling scratches: This type of defect mainly comes from the head and tail of each coil of steel strip, and is caused by the tension of the coil shaft during rolling. Especially for thick materials with a bright surface of BA and a thickness of over 1.2mm, this defect is particularly obvious. In response to such defects, during the rolling process, it is necessary to pay close attention to the monitoring of the tension at the head and tail coiling of the steel strip. When rolling thick materials, it is essential to ensure the stability of tension control during the tensioning or de-tensioning of the coiling shaft.

(2) Cold rolling pits or marks: Under the operating conditions of high rolling force, if the surface layer of the work roll shows fatigue and meat shedding or foreign small particles adhere to the surface of the work roll, such defects will occur. First of all, it is necessary to ensure that the surface hardness of the purchased work rolls meets the process requirements. During the daily production and operation, the grinding amount of the fatigue layer on the surface of the work rolls must be well controlled between the roller grinding machines (RGS), while for the 20-roll cold rolling mill production line (CRM), the total number of rolling passes for a pair of new work rolls used in the rolling process must be strictly controlled.

(3) Taper roller indentation (tire tread) : This type of defect is mainly caused by damage to the surface of an intermediate roller. The grinding amount of the fatigue layer of one intermediate roller/roller surface between the double-roller grinding machines (RGS) must be well controlled. The 20-roll cold rolling mill production line (CRM) must rationally allocate the reduction amount and rolling tension matching for each pass based on the differences in the roll system of the frame and the material characteristics of different steel grades, so as to effectively prevent excessive rolling force from damaging the roll surface.

(4) Vibration marks: The grinding process between roller grinding machines (RGS) must be strictly controlled to avoid the formation of hidden vibration marks on the roller surfaces of the work roller, the first intermediate roller, the second intermediate roller and the support roller, which will eventually cause vibration marks of different periods on the surface of the steel strip. The 20-roll cold rolling mill production line (CRM) must pay attention to preventing slippage in the first few passes during rolling, which may cause continuous vibration marks on the surface of the steel strip that cannot be completely eliminated until the last pass.

(5) Scratching of the oil squeezing roller: Due to the fact that the 20-high rolling mill is often in a high-speed operation state during the rolling process, scratching defects are very likely to occur. For the oil squeezing roller system, we must strictly control the regular update and maintenance of the oil squeezing roller wiper. And pay attention to the matching of the roller diameter of the oil squeezing roller and the regrinding quality of the roller surface (such as the hardness and convexity requirements of the roller surface of the wool roller).

(6) Stripes or whitening on the plate surface: Mainly control the reduction amount of the first few passes and the last three passes of each roll, the amount of cooling oil, and the rolling speed. Use cold-rolling oil with appropriate composition.

(7) Oil stain defects: Strengthen the control of the oil squeezing effect of the oil squeezing roller wiper and set the pressure of the wiper reasonably; When choosing the type of cold-rolling rolling oil, special attention should be paid to the annealing and cleaning performance in the characteristics of the rolling oil.

(8) Mandrel winding crease: Especially for specifications with a target thickness of less than 1.0mm, the appropriate rolling unit tension should be adopted based on the material properties of the steel coil and the steel sleeve should be 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 surface temperature of the steel strip. Pay attention to the surface roughness of the intermediate backing paper used in the last pass of the rolling mill (the quality characteristics of the backing paper).

(10) Poor plate shape: Intuitively speaking, plate shape refers to the warpage degree of the plate and strip material. In essence, it is the distribution of residual stress inside the plate and strip material. As long as there is residual stress inside the sheet and strip material, it is considered a poor sheet shape. If the residual stress is insufficient to cause warping of the plate and strip, it is called a "potential" plate shape defect. If residual stress causes instability of the plate strip and leads to warping, it is called "apparent" plate shape defect. Common plate shape defects include various forms such as edge waves, middle waves, single-sided waves, double-rib waves and composite waves. They are mainly caused by uneven extension of different parts of the strip during the rolling process, which generates internal stress. It is well known that the main factors affecting the shape of the plate include the following aspects: the change of rolling force, the change of the convexity of the incoming sheet, the convexity of the original roll, the width of the plate, the tension, the contact state of the roll, the change of the thermal convexity of the roll, etc. To obtain high-quality rolled strip materials, it is necessary to adjust the roll gap of the rolls at any time to suit the plate convexity of the incoming material and compensate for the influence of various factors on the roll gap. For strips of different widths, thicknesses and alloys, only with one optimal convexity can the rolls produce the ideal target plate shape. Therefore, the essence of plate shape control is the control of the gap between the bearing rolls. Unlike thickness control which only needs to control the opening accuracy at the midpoint of the gap, plate shape control must control the shape of the entire gap within the width span of the rolled piece. Based on this, for plates of different specifications and with different material properties, it is necessary to consider the reasonable selection of the taper or convex degree of different roll series in the entire roll system of the 20-roll mill to ensure the accuracy of plate shape control during rolling.

5 Conclusion

Due to the complexity of the entire system's operating conditions of the 20-high rolling mill, the diversity of the material properties of stainless steel coils, and the differences in the surface requirements of customer orders, and the fact that many product defects in the production process of stainless steel plates originate from the cold rolling process section of the 20-high rolling mill, How to rationally match and optimize the equipment performance and process parameters of the 20-high rolling mill to effectively reduce the proportion of major defects in the cold rolling process section has always been the main pursuit goal in the production of stainless steel plates.