In the process of hot strip production, if continuous, dense, uniform and adhesive oxide film is formed on the roll surface of finishing mill, not only the friction coefficient between roll and rolling stock can be reduced, but also the dimensional accuracy and surface quality of rolling stock can be improved. However, due to the low rotation speed of the fi-f4 stand roll of the finishing mill, the oxide film forms on the roll surface quickly at high temperature. With the increase of rolling time, the oxide film peels off in the rolling process, resulting in the pitted scale defects on the surface of hot rolled coil, which has become a difficult problem to control and solve.
Title：Relationship between surface quality of working rolls and surface defect of hot rolling strip
Keywords：oxide film; scale defect ; working rolls control measures
Morphology and causes of scale defects
The scale formed on the surface of hot rolled products in the finishing rolling area due to the peeling off of oxide film on the roll usually becomes the third scale. The work roll of the front stand of finishing mill works under the conditions of high temperature, high speed, large reduction and sudden cooling and heating. The oxide film on the roll surface periodically bears huge alternating stress. After reaching a certain fatigue limit, the microcracks in the oxide film on the roll surface generate and expand at the crack source. When the crack size grows to a certain extent, under the strong shear stress between the work roll and the hot rolled strip, the oxide film on the roll surface falls off and adheres to the surface of the strip steel, which is rolled to the surface of the strip steel in the subsequent stand and forms oxide scale defects.
Therefore, the surface quality of hot rolled strip depends on the macromorphology of oxide film. The influence degree of roll surface roughness and strip surface quality is as follows:
During the production of 2.0 mm export material s235 by hot strip mill, pitted scale defects appeared on the surface of hot rolled strip in finishing rolling area, which could not be removed after pickling, resulting in a large amount of modification. It can be seen from the surface detection equipment that the change trend of oxide scale morphology on the surface of strip steel in a rolling cycle is shown in the figure below:
It can be seen that the density of scale is high at the initial stage of formation, and the distribution of scale is pitted. With the rolling process, the degree of scale is gradually reduced, and finally the scale is banded.
3 Defect analysis
3.1 chemical composition of work roll and strip steel
The high speed steel rolls used in FI and F4 of finishing mill are made of high chromium cast iron, and the chemical composition is shown in Table 2.
In hot rolled products, s235 steel is prone to such scale defects. One coil is selected as the research object, and its chemical composition is shown in Table 3.
It can be seen from table 3 that Cr, Ni and other elements are not contained in the composition of strip steel with scale defects.
3.2 selection of defect samples
In the rolling combination unit 6838 + 6837 + 6815 + 6847 + 6844, s235 is concentrated in 6815 unit, the 65 ~ 84 coils are 2.0x1255mm s235 steel, the 20 coils all produce scale defects. Take samples from the defective parts of 92948928 steel coil, and take samples from the parts without sheet defects.
3.3 defect analysis results of samples
The results of electron probe microanalysis are shown in FIG. 10 and FIG.11
It can be seen from the above two figures that only Fe. 0 exists on the surface of the sample without scale defect, but no CR exists; In the samples with oxide scale, there are not only fe.0 but also cr.si and Mn in the defects.
4.Control methods and measures
4.1 reasonable rolling plan
There is a process for the formation of oxide film on work roll, and the oxide film is also changing after the formation: a better rolling plan can also pre control the formation and change of oxide film. In the same rolling cycle, the narrow to wide start rolling specifications are not only beneficial to shape control, but also form a good overall roll oxide film. The change of roll shape supports the formation of oxide film. After a period of ironing, a thin oxide film is formed on the whole roll surface. If there is no sudden change of working conditions in the rolling process, rolling to about 45km (subject to the final stand) is the best period of roll surface state. Therefore, varieties with high surface requirements in production are basically arranged to be rolled during this period, and high strength steel should not be rolled in front of the plan.
In a word, when arranging the rolling plan, special attention should be paid to the reasonable transition of strip thickness, width, strength, finishing temperature and the control of rolling kilometers.
4.2 stable rolling rhythm
In order to establish an ideal oxide film, the key lies in the temperature control of the roll body at the initial stage of the work roll, so it is necessary to formulate a strict roll ironing system. Hot rolling must be carried out in strict accordance with the regulations: when the first five pieces of steel are rolled after roll change, the rolling time interval between the two pieces of steel is about 5min, and the maximum flow of cooling water for the front stand roll is maintained to ensure the smooth preheating of the roll; When 5 pieces of steel are rolled after rolling, the rolling time interval between two pieces of steel is 3-4 min. the cooling water flow of the front stand roll should be reduced appropriately, and the roll temperature should be increased appropriately to accelerate the formation of roll oxide film; After rolling the I1 steel, the amount of cooling water is restored to the maximum, so as to prevent the oxide film from further thickening and peeling off. During hot rolling, the rolling rhythm should be slow, so as to ensure that the oxide film on the roll surface is well established.
In addition, when there are too many high temperature coils arranged in the same rolling plan, the cooling effect of rolls should be adjusted by controlling the rolling rhythm.
4.3 reasonable load distribution principle
The work roll of finishing mill works under the conditions of high temperature, high speed, large reduction and sudden cooling and heating. The oxide film on the roll surface bears huge alternating stress periodically. If the load distribution of rolling mill F1 ~ F6 is unreasonable and the load of one or several stands is too large, the oxide film on the work roll surface will reach the fatigue limit, and cracks will appear on the oxide film. When the crack size grows to a certain extent, the fatigue limit of the oxide film on the roll surface will be reduced, Under the strong shear stress between the work roll and the hot strip, the oxide film on the roll surface will peel off. Therefore, the reasonable distribution of load is very important for the protection of oxide film. If it is found that the load of a stand is too large in production, the load of the stand can be appropriately distributed to other stands, or a part of the load can be adjusted to the roughing mill to reduce the shear damage to the oxide film on the roll surface.
4.4 optimize roll change mode
The replaced work roll needs to be grinded. For the roll with burn, steel sticking, tail flick and mesh crack, the amount of grinding should be increased until the crack and fatigue layer are completely grinded (in theory, a small amount of grinding should be added after the grinding is clean), otherwise it will be the hidden trouble of damaging the oxide film. Due to the lack of grinding amount, the roll will bring quality problems, and the roll is easy to aging and fatigue, resulting in surface defects of strip steel.
In addition, any abnormal rolling may cause damage to the roll, and the roll with water shortage, water cut-off, steel sticking and overlapping, and foreign body pressing must be replaced immediately.
After the steel jam accident, the cooling water of the roll must be shut down immediately, and the roll must leave the hot strip as soon as possible to replace the roll. The roller can naturally prevent the hot crack from chilling.
The defects and cracks on the surface of the roll must be cleaned up. It is necessary to add a detection of the bonding layer before the machine is used again. When it is used again, the defect parts shall be tracked and detected at least three times.
4.5 cooling water system
During the hot rolling process, the instantaneous surface temperature of the work roll is 500C – 700, and the temperature of the roll body exceeds 60C. Key factors affecting the growth of oxide film
The higher the roller body temperature is, the easier the oxide film is to grow and thicken, and it is easy to crack and peel off. Therefore, the cooling condition of roll should be improved and controlled to form uniform and dense oxide film with good adhesion on roll surface. Especially for F2 and F3 stands, the cooling intensity of roll should be increased appropriately to control the roll temperature at about 60 ~ 65C. At the same time, the cooling water should be increased in the middle of the roll body to reduce the temperature difference. In addition, the rolling length and roll changing period should be reasonably determined in combination with the appearance and quality of the oxide film on the roll surface, so as to avoid the influence of the oxide film falling off on the surface quality of the strip.
The cooling water nozzle of the roll should be checked every time when changing the roll to ensure the smoothness of the nozzle. If there is blockage, it should be replaced immediately to ensure the normal operation of the cooling water of the roll. At the same time, the angle and distance of water nozzle and the effect of water cutting board meet the standard requirements. The roll temperature should be controlled at about 60-65c after the roll is removed from the machine (within 30 minutes).