The cold rolling mill adopts a single-stand, reversible, six-high rolling mill. During the production process, abnormal shutdowns of the production line due to the fall of the roll surface occur from time to time. Abnormal shutdown accidents not only increase production and manufacturing costs but also affect and disrupt the normal production rhythm, resulting in unbalanced process capacity and affecting customer delivery. In order to solve the problem of roll surface spalling, Longwei Company analyzed the force of the roll, combined with the factory's on-site process conditions and actual operating environment, and found out the reasons for the roll burst and put forward effective preventive measures. , And achieved good results.
1. Roll burst phenomenon
During the production and operation of the roll, during the roll change process, and during the placement process after the roll change, local deep bursts, large surface shedding, and cracks of different degrees appeared, as shown in Figure 1~Figure 4.
(1) Burst roll occurred during the production process. In the normal production process of the steel rolling workshop, there was a band-breaking accident accompanied by severe abnormal noise. There was no obvious rule to follow when the accident occurred. When producing different materials, different thickness specifications, different passes and different thicknesses. In the rolling stage, there was an irregular and sudden shutdown accident. At the scene of the accident, severe cracking of the roll body and partial surface peeling were seen. Among them, the phenomenon of middle roll explosion and surface shedding is particularly obvious, because the middle roll body is peeled off in a large area, which not only causes the middle roll to be directly scrapped, but also the accident also causes different degrees of damage to the work roll, causing the online rolling product to be cut and divided. Roll, resulting in a lot of lost material loss.
(2) The roll breaks after being changed off the machine. In accordance with the requirements of the rolling process and roll change system of the cold rolling mill, the rolls must be removed from the machine for maintenance and grinding after they are rolled to a certain number (including the number of coils, kilometers, passes, and tonnage), so that they can be paired, disassembled, and then prepared. use. Sometimes the rolls are in the process of changing rolls
The roll body may burst in the middle, or the roll that has just been replaced, and a light or heavy explosion sound can be heard, cracking and peeling. In severe cases, the fragments of the roll body can explode up to several meters away, which not only causes the roll to be directly exposed. Discarded, and there is also a safety hazard that the flying film may hurt people.
2. Analysis of the causes of roll burst
(1) Analysis of force situation. During the rolling process, the rolls of cold rolling mills mainly bear bending stress, fatigue stress and thermal shock fatigue stress.
① Bending stress. When the reversible six-high HC rolling mill is rolling, the positive bending force: maximum 300KN, the negative bending force: maximum 200KN, the maximum rolling force is 10MN, the rolling speed is 0m/s~10m/s, and the roll drawing distance is 200mm. Because the work rolls, intermediate rolls and backup rolls are tightly pressed together, the rolling pressure is evenly distributed along the rolling piece, and the rolling forces on the rolls are evenly distributed, which is not the main cause of roll spalling within the safe rolling load. However, if the pumping volume of the intermediate roll is too large, or when the load is too large, the contact area between the intermediate roll head and the work roll and the support roll will produce local bending stress concentration, and the edge effect of the emulsion will also be in this area. Accumulation of metal slag and impurities occurs, resulting in increased stress concentration, resulting in local cracks. As the amount of rolled steel increases, roll cracks and surface bursts are caused. The crack propagation mostly occurs when the support roll shoulder falls off and the middle roll head Burst and fall off.
② Fatigue stress. During the rolling process of the roll, each revolution of the roll is subjected to tensile and compressive stresses in the middle of the roll body where the stress is highly concentrated. They are constantly changing cyclically, and the place where the stress concentration of the roll body has a larger fatigue stress. After a certain cycle, cracks are generated at the sharp corners of the inclusions and propagate along the direction of the inclusions and stress, which can cause the roll surface to peel off, which is an influencing factor for cold rolling burst.
③ Thermal shock fatigue stress. During the rolling process, the roll deformation zone and the cooling zone are constantly changing alternately, that is, the roll is subjected to constant heating and cooling. In this operating environment, the roll is subject to long-term alternating thermal stress in the cooling zone and the deformation heating zone, which is easy to Initial micro-cracks appear on the surface of the roll, and this kind of micro-cracks will continue to intensify, which is a factor that causes the roll surface to peel off.
(2) Actual process conditions.
① Insufficient cooling. The temperature of the roll in the accident was high, and the temperature was uneven, locally reaching 300°C. The inspection found that the emulsion nozzles of the rolling mill had uneven injection flow and pressure, and many nozzles were clogged to varying degrees. Because the cold rolling of the roll by the rolling mill emulsion is uneven and insufficient, the heat of the rolling material deformation during rolling cannot be completely released, which will increase the temperature of the roll and cause axial opening cracks. In addition, the rolling mill emulsion temperature is too low, the concentration is too low, and the cleanliness of the emulsion is not up to standard are also factors that affect the roll cracks.
②Technical skidding. When a roll burst accident occurs, the rolling mill occasionally appears to be slippery. Due to the unreasonable rolling process and the imbalance of the front and rear tension, the slip between the work roll surface and the rolled material is caused. Severe vibration and frictional heat caused by process slippage will be transferred to the work.
The roller and the intermediate roller make the temperature of the roller surface increase rapidly, which causes cracks on the roller surface.
(3) Actual production situation.
Damaged rolls caused by production accidents. 90% of roll bursts are accompanied by rolling broken strip accidents. Although it is impossible to accurately determine the relationship between the product strip and the roll burst, the roll break accident during rolling will definitely cause the roll to be injured. When the strip is broken, the temperature of the roll changes drastically, and cracks are inevitable, and the depth is deep, localized, and the depth and opening are large; when the strip is broken, piles of steel and rolling will occur. Due to the inertia of the high-speed operation of the rolls, it is instant The impact and power accumulation are enough to cause severe damage to the roll. When the belt is broken and steel sticking occurs, the impact on the work roll and the intermediate roll is fatal. Sticking steel causes indentation on the roll surface, which will be directly transmitted to all roll surfaces. If the work roll, intermediate roll, and support roll are not found and replaced in time, after a certain rolling cycle, the indentation will be in each roll. Continuous transfer and copying, and the formation of slight cracks at the indentation will expand and intensify, and finally cause the working layer of the roll body to fall off and the residual layer to peel off.
(4) Roll problem.
①The hardness exceeds the standard. The hardness of the working surface of the roll is the main quality index of the roll. Although the hardness index determines the service life of the roll, if the hardness of the roll itself cannot match the index of the work roll, intermediate roll, and support roll, it will still cause injury to the roll. The hardness matching of the rolls of Longwei Company is: working hardness 90~95HSD, middle spoke hardness 75~80HSD, support hardness 60~65HSD. After the new roll is paired with roll diameter and hardness, the hardness of the roll will change locally during use. This causes local cracks and peeling of the roll itself, and at the same time, the roll will also induce local bursts of adjacent rolls.
② Maintenance and grinding of rolls. After the roll is off the assembly line, the fatigue layer on the roll surface and the surface crack layer will form defect residues due to incomplete grinding. When the defective roll is used again on the machine, under the action of periodic fatigue stress, micro-cracks will quickly appear and cause damage. Continue to expand until a certain critical depth or transition layer, laying hidden dangers for the roll surface peeling.
3. Improvement measures to avoid roll bursting
3.1. Ensure the grinding process and maintenance system of the roll
Establish strict roll service cycle, roll maintenance and grinding system guarantees. According to the material and performance characteristics of the rolls, a reasonable roll maintenance system shall be established in combination with the actual production conditions and process conditions to ensure the process requirements for the rolls to be used on the line. In general, work rolls, intermediate rolls, and support rolls can be maintained by referring to (Table 1).
In order to prevent the roller from cleaning its own defects incompletely and getting on the machine with illness, the repair and grinding amount of the injured roller can be increased to 0.40mm~0.55mm, and the flaw detection after grinding is strengthened to ensure that the roller eliminates internal stress and is thorough. Remove micro cracks.
3.2. Strictly pair the rolls
The pairing of support rollers should ensure the service life and surface quality, including the paired use of support roller bearings, to ensure the stable operation of the support rollers. The matching between the work roll and the intermediate roll itself and the matching between the work roll and the intermediate roll are particularly important. The pairing of rolls must meet the roll diameter, roll surface hardness, roll life cycle and other indicators, especially the injured roll, initial roll, and end period. Paired use of rolls.
3.3. Ensure the operation of emulsion process
Taking into account the cooling function and lubrication function of the rolling mill emulsion, due to environmental protection requirements, the full-cycle, emission-free emulsion system has been gradually promoted, and the service cycle of the emulsion has been greatly extended. The cleanliness of the emulsion must be ensured and the concentration of the emulsion must be monitored in real time , Cleanliness and other physical and chemical indicators. Ensure the realization of the effect of the emulsion on the cleaning, lubrication, and cold rolling of the rolling mill and rolls, and avoid the contamination of the emulsion by the rolling mill AGC hydraulic oil, oil and gas lubricating oil, and maintenance oil.
3.4. Implement the roll change system
According to the material and thickness of the rolled products, the rolling process implemented is different, and the corresponding roll change system is formulated. Generally, refer to (Table 2) to implement
3.5. Adhere to the roll preheating system
After the roll is changed, ensure the roll preheating time, preheating temperature, and preheating pressure. Give sufficient hot roll time before rolling, generally 30MIN~40MIN, to ensure a certain hot roll pressure, generally 4MN~5MN, to meet a certain temperature of the hot roll, the temperature requirement of the emulsion is generally: summer, 45℃ ~50 ℃; in winter, 50℃ ~55℃.
3.6. Optimize the rolling process
The products rolled by the single-stand rolling mill are getting thinner and thinner, and the production batches of the same specification are small, and even the production is organized in batches according to the coils, resulting in large differences in the use and quality of the products before and after, and the rolling process needs to be adjusted in time. Organize production according to the quality requirements of the next process and the end user. According to the user's different focus on the shape, size, mechanical properties, and chemical composition requirements of the user, through high-speed rolling, high-reduction rolling, and high-tension rolling Wait for the optimization and adjustment of the rolling process.
3.7. Establish a linkage mechanism for vertical and horizontal integration
The steel rolling process must respond to the upstream raw material changes in a timely manner, and the technical requirements of the next process must be transmitted in time; the roll usage and online index information must be shared with the roll repair shop in real time, and the operation maintenance and accident handling must be communicated with the maintenance shop. The vertical and horizontal linkage mechanism ensures the efficient and smooth running of the rolling mill.
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