High-speed steel rolls are high-carbon high-alloy steels containing a large amount of tungsten, molybdenum, chromium, vanadium and other elements in the working layer. They have the characteristics of high hardness, excellent wear resistance, good hardenability and high red hardness, and are widely used in heat transfer rolling mills. Products rolled with high-speed steel rolls have the advantages of high quality, good precision, and high surface smoothness. The use effect of the roll depends on the structure of the roll, and the structure is determined by the chemical composition. The chemical composition of the high-speed steel roll is different, and the structure and performance of the roll are different.
1. Characteristics of high-speed steel rolls
1.1 Chemical composition control of the working layer
In the national standard, the scope of the chemical composition of high-speed steel rolls is very wide. When various roll manufacturers produce high-speed steel rolls, the content of alloy elements used in the chemical composition varies greatly when implementing national standards, and their manufacturing costs are completely different. Performance A big difference. Therefore, the chemical composition of the working layer of the roll must be strictly controlled and tested.
Chromium (Cr) first forms carbides to increase the stability of the matrix. Increasing the chromium content of high-speed steel rolls can produce higher hardenability, so that the hardness from the surface of the roll body to the inside of the working layer hardly decreases, thus ensuring that the roll is from outside to inside. It has the same good wear resistance; but if the chromium content is too high, the thermal stability and red hardness will be reduced, and segregation will easily occur.
Tungsten/molybdenum (W/Mo) can refine grains and improve the hardness and redness of high-speed steel.
Vanadium (V) can form carbide VC. VC has high hardness and fine grains. It is mainly distributed in spherical or near-spherical form on the matrix, which can improve the impact toughness, hardness and wear resistance of the material.
Niobium (Nb) is similar to vanadium in that niobium has a greater affinity with carbon and is more likely to form carbides.
1.2 Roll surface oxide film
The surface quality of strip steel is an important indicator of strip steel quality. One of the most important factors affecting the surface quality of strip steel is the oxide film on the surface of the roll. If the peeled oxide film is pressed into the surface layer of the strip steel, it will not only form defects such as scale indentation but will also have a serious impact on the surface quality of downstream cold-rolled sheets.
1.2.1 Mechanism of oxide film generation
The oxide film on the roll surface is formed and grown by the diffusion of iron ions and oxygen in the roll surface under the conditions of high temperature and water vapour formed by the roll surface, strip steel and roll cooling water. Oxygen molecules in the atmosphere are adsorbed on the surface of the roll surface and dissociated to obtain electrons to generate oxygen ions and then meet with iron ions in the matrix of the roll surface to form an oxide film. The oxygen ions continue to diffuse into the roll. At the same time, the iron ions and Electrons are released from the iron surface and transported to the surface of the oxide film, and oxygen ions and iron ions meet somewhere in the oxide film to form new oxides.
1.2.2 Macroscopic morphology of oxide film
The thickness of the oxide film determines its macroscopic appearance, and the macroscopic appearance of the oxide film determines the surface quality of the hot-rolled strip steel; the ideal oxide film on the roll surface will not fall off after a long period of rolling, and the oxide film is resistant to shearing. The strength and the bonding strength with the matrix decrease with the thickness of the oxide film, and its thickness exceeds 2 μm. Under the repeated action of rolling force, the oxide film initiates microcracks along the carbide, which makes the oxide film appear abnormal and fall off.
In order to establish an ideal oxide film, the key lies in the temperature control of the roll body at the beginning of rolling after the work rolls are put on the machine, so a strict hot roll system and a cooling water volume system for the work rolls must be established; Gentle preheating to prevent the oxide film from further thickening and peeling off, and the rolling rhythm during the hot roll should be slow to ensure that the oxide film on the roll surface is well established.
1.2.3 Evaluation of Surface Quality
In production, since it is impossible to carry out on-site thermal sampling research on the rolls, according to production experience, the surface quality of the rolled rolls is qualitatively evaluated through the oxide film on the roll surface, on-line wear and the temperature change of the roll body.
a. Oxide film level
Grade A: The oxidation film of the off-machine is rated as I and II, the surface is smooth and shiny, the oxide film is dense, black or brown, or partially rough, without hand feeling;
Grade B: The oxidation film of the machine is rated as Ⅲ or Ⅳ, and the rough surface is obviously or slightly peeled off;
Grade C: The oxidation film of the machine is rated as V and VI, with “comet”-shaped peeling or peeling area expanding, and the rough surface is exposed;
Grade D: The oxide film on the lower machine is rated as grade VII, with large areas of peeling off, and the peeled surface is in pieces or the entire roll surface is peeled off.
b. Online wear
Grade A: The roller body passes through the steel area, and the wear is uniform and smooth;
Grade B: In the area where the roll body passes through the steel, the local wear is uneven, and the wear difference between the highest point and the lowest point of the local area is not more than 0.02 mm;
Grade C: In the area where the roll body passes through the steel, the local wear is uneven, and the wear difference between the highest point and the lowest point of the local area is greater than 0.02 mm and not greater than 0.03 mm;
Grade D: In the area where the roll passes through the steel, the wear difference between the highest point and the lowest point of the local area is greater than 0.03 mm.
c. Change the trend of roll body temperature of the lower roll
Grade A: The temperature curve is smooth, without abnormal high and low points;
Grade B: In the area where the roll passes through the steel, the temperature curve has a high and low abnormal point;
Grade C: In the area where the roll body passes through the steel, the temperature curve has no more than 2 high and low abnormal points;
Grade D: In the area where the roll passes through the steel, the temperature curve has 3 or more high and low abnormal points.
1.2.4 Maintenance of oxide film
It is very important to maintain the temperature of the roll body on the line to avoid rapid cooling and heating of the roll. Therefore, strengthening the strong cooling of the work roll and shortening the time when the roll surface is in a high-temperature state are important means to control the thickness of the oxide film on the roll surface.
a. Roll peripheral temperature control
After the roller surface comes out of the contact arc with the strip, in order to shorten the time that the roller surface is in a high-temperature state, the temperature must be lowered from 700 G to below 300 G as soon as possible to prevent further oxidation. The cooling water device with two rows of nozzles on the entrance side of the roll and three rows of nozzles on the exit side increases the cooling water volume on the exit side considers the optimization of the wiper to discharge the residual cooling water and avoids uneven cooling of the roll body.
b. Control of roll axial temperature
The different widths of the strip steel and the roll during rolling, cause uneven temperature distribution of the roll in the axial direction, which directly affects the maintenance of the oxide film of the work roll and the control of the strip section, and there is a local thick oxide film or Shedding phenomenon. By optimizing the cooling water distribution of the work rolls, the amount of cooling water in the middle of the work rolls is increased, the axial temperature of the roll surface is obviously evenly distributed, and the thickness of the oxide film tends to be normal; in addition, the smoothness of the roll cooling water nozzles affects the roll Another main reason for the uniformity of temperature in the width direction, so the roll cooling water nozzle should be checked every time the roll is changed, and if there is any blockage, it should be replaced immediately to ensure the normal input of the roll cooling water.
c. Optimizing mill load distribution
In the process of production practice, the load distribution in the F2 stand is the largest, the rolling force of the F2 stand can be appropriately reduced, and the rolling force of the F3 and F4 stands can be increased. However, it is necessary to consider that the temperature drop of the slab on the F4 stand will cause the rolls, The vibration of the rolling mill leads to the shedding or poor formation of the oxide film, so the reasonable distribution of load is very important for the protection of the oxide film.
d. Rolling lubrication
Rolling lubrication can reduce the friction between the strip and the roll oxide film and the rolling force, and reduce the shear stress caused by friction.
e. Roll grinding
Reduce the continuous feed and end feed of the grinding machine to avoid small scratches or tool marks on the roller body caused by grinding and grinding wheels, which will affect the quality and stability of the oxide film formation.
2. Detection and tracking of high-speed steel rolls
Compared with high-chromium iron rolls, the finish-rolled high-speed steel rolls have the advantages of high hardness and mechanical properties, reflecting excellent wear resistance and roll shape retention. At the current stage, due to the differences in the working conditions and effects of the high-speed steel rolls of different manufacturers, in addition to the control of the chemical composition of the working layer of the rolls, it is particularly important to check the acceptance of new rolls and the inspection and tracking after use.
2.1 Acceptance of new rolls
In addition to the mechanical performance items in the factory entry agreement, the acceptance of new rolls also includes non-destructive testing and dimensional acceptance. The non-destructive testing is mainly to detect the surface, internal defects and hardness of the roll. The guarantee of no defects on the surface and the interior makes the new roll have good condition before use, and the hardness value ensures that the high-speed steel roll has good performance; the size acceptance is mainly the bearing assembly part The accuracy, thickness and symmetry of the transmission flat head should meet the requirements to avoid bearing damage or running out of the outer circle and rolling beating.
2.2 Tracking detection after getting off the plane
Eddy current flaw detection should be carried out after each roll surface grinding. Ultrasonic testing should be carried out for abnormal eddy current testing or rolling mill accidents; after a certain thickness of the working layer of high-speed steel rolls is used, monitoring and testing of the hardness of the bonding layer and the roll body should be carried out & oxide film evaluation and data tracking should be carried out after each roll change, timely analysis and quick response in case of abnormal situation, solve the problem of rolling mill working conditions, and avoid abnormal rolling of the roll.
3. Conclusion
The use of high-speed steel rolls can increase production, reduce the number of roll grinding repairs, extend the number of rolling kilometres, and ultimately reduce costs and improve production and quality. Through the research on the use of technology of high-speed steel work rolls, the importance of high-speed steel performance and oxide film is understood, and the good performance of high-speed steel rolls can be better played by combining the acceptance of new rolls and the detection and tracking after off-machine; however, the performance of high-speed steel from different manufacturers is difference, and there is still a lot of research work to be done to successfully promote the application of high-speed steel work rolls.
Article source: Modern Metallurgy – “Research on the Technology of Using High-Speed Steel Rolls”
