Analysis and Research on Wear Law of Section Steel Roll

The wear of the profile steel pass is directly related to the production efficiency of the rolling line and the quality of the product. At the same time, it has a great impact on the adjustment of the mill profile and the control of the negative difference. Therefore, by understanding and mastering the wear pattern of each pass in the rolling process, the material type can be adjusted and the rolling groove replaced in time, and corresponding measures can be taken to achieve the purpose of stabilizing production, improving the rolling groove tonnage and product quality. The results show that: in the process of rolling angle steel, measures such as improving the material of the roll, modifying the finished pass, optimizing the slope of the side wall of the rolling groove before the finished product, improving the processing accuracy of the roll, formulating a reasonable rolling groove tonnage and reduction system, etc. Reducing the degree of wear and unevenness of the rolling groove has played a key role, so as to achieve the purpose of improving the rolling tonnage and the quality of the finished product.

The roll is a direct tool for metal deformation during the rolling process. Therefore, grasping the law of roll wear and taking corresponding measures can achieve the purpose of reducing roll consumption, increasing operation rate, improving product quality, and reducing employee labor intensity. Because of the special-shaped pass used in the rolling of section steel, the force of the rolling groove is uneven, and the wear conditions are also different. The study of the wear law of the section steel rolling groove is of great significance for guiding production.

Research on the Law of Roll Wear

Since the profile operation area of the No. 1 Steel Rolling Plant of HBIS was put into production, it has successfully developed 8 # ～ 20 # angle steel (national standard, European standard, etc.), some channel steel specifications, round steel φ 50 mm～φ110 mm and some mines. U-shaped steel specifications are used. In this paper, when studying the wear of rolls, the angle steel specifications are selected for follow-up investigation. The wear data collected through production over a period of time reflects some of the relationship between roll wear and rolling tonnage, and also reflects the rough rolling pass, the butterfly hole before the finished product, and the finished hole wear with the use of the rolling groove. The location and amount of wear.

Wear of rough rolling pass:

Through the measurement of the wear of the off-line rolls, the wear of each rack of rough rolling is relatively uniform, and it is basically proportional to the rolling tonnage, as shown in Figure 1.

Wear condition of butterfly hole:

The angle steel finish-rolled pass adopts butterfly hole as shown in Figure 2. Due to the different work roll diameters of the rolls at each point in the pass, the pressure of the rolled piece at each point in the pass is different, and the amount of roll wear varies greatly. Therefore, unevenness is the most obvious feature of roll wear in section steel rolling.

Figure 2 shows the roll matching diagram of the angle steel butterfly hole. The roll wear was analyzed and compared by the opportunity of the roll reloading, and the roll wear curve in Fig. 3 was formed.

The wear of the upper and lower rollers in Figure 3 shows that: the wear of the angle steel butterfly pass shows great unevenness.

Due to the different rolling specifications and pass design dimensions, the wear condition of each pass is also different. But the general trend is that as the diameter of the work roll at each point of the pass decreases, the amount of wear increases. The maximum wear of the hole pattern occurs at the top corner, the upper roller leg end of the lower opening butterfly hole and the lower roller leg end of the upper opening have a larger amount of wear due to the pressure brought by the side wall, other The straight line and arc parts have relatively little wear due to the close hole design.

Finished pass wear:

In the angle steel rolling process, the deformation of the finished product is the most severe, so the wear of the finished groove is very uneven, which affects the rolling tonnage and product quality. Table 1 shows the wear of the finished roll of 125 gauge steel.

From the above data, it can be seen that the wear of the upper roller in the finished angle steel hole is relatively small, and the lower roller exhibits a law of wear that is the largest at the leg end from the vertex angle of 1/2 to 2/3 of the leg width, and then gradually decreases. The wear of the finished hole is mainly concentrated in the severely deformed arc area. Since the top angle of the previous rolling piece is close to 90 °, the wear of the top corner is not large.

Wear analysis and countermeasures

The roll hardness is not uniform.

The heat-treated roll has a certain depth of working layer, and the relative hardness difference is not large. However, the pass of the section steel is a special-shaped hole, and the depth of the rolling groove is relatively large. In addition, the reason for the need to configure the upper and lower pressures for the rolling of the section steel. Layer, and the other part is in the working layer, causing the hardness drop of the entire rolling groove to increase, and the unevenness of wear is more obvious.

Laser surface strengthening technology has developed rapidly in recent years. Its most important feature is the extremely fast heating and cooling speed, which is conducive to the remarkable refinement of the crystal grains of the quenched layer, strengthens the metal surface layer, and makes the metal surface have high wear resistance. By adopting this technology, the hardness of each point of the pass is nearly the same, which greatly reduces the uneven wear caused by the uneven hardness of the roll, and the tonnage of the groove is increased to more than twice the original.

Unreasonable hole design

Angle steel rolling mostly uses butterfly holes. If the parameters are selected unreasonably, it will accelerate the wear of the rolling groove, increase the difficulty of adjustment, and cause unstable production. Due to the special-shaped section, during the rolling process, the rolled piece does not contact the roll at the same time, resulting in not all parts of the rolled piece sliding forward. With the increase of the roll diameter, the forward slip value decreases, and when the speed of the roll exceeds the speed of the rolling piece, the backward slip occurs. Such different sliding values result in poor friction, resulting in uneven wear of the hole pattern. If the pass design parameters are unreasonably selected, the clamping force of the sidewall of the rolled piece will increase, and the wear will be accelerated, while the difficulty of degrooving will occur, which will cause the rolled piece to warp at the exit and cause a production accident. In the butterfly hole design, under normal circumstances, the side wall slope is gradually reduced from 20% to 10%, the arc of each frame side wall is appropriately increased, and the design concept of large slope and large arc is adopted. By optimizing these parameters, the degrooving of the rolled piece is easier, and the arc fullness of the leg end is better, which not only reduces the wear of the groove, but also improves the product quality and reduces the process accidents. There is a big difference between the maximum and minimum diameters of the bottom rolling groove of the finished hole. The rolling piece slides relative to the metal at the end of the leg at the inner arc of the pass, which causes the wear rate of the rolling groove to increase, and the wear at the arc is the fastest, and As the rolling tonnage increases, the wear intensifies, and the metal accumulates at the inner corners of the arc, which is not conducive to negative differential rolling. According to this feature, we can make appropriate modifications to the leg of the finished angle steel hole as shown in Figure 4.

Among them, Hb is the standard leg thickness, Hy is the modified waist thickness, and Hz is the increase of the modified waist thickness, so you get

Hy= Hb+ Hz, according to the measured data in Table 1, the range of Hz= 0.2 to 0.5 mm can be obtained. Through the modification of the pass, the problem of the reduction of tonnage caused by the wear of the rolling groove, the uneven thickness of the rolled leg and the difficulty of controlling the negative difference are solved.

Other means:

Other methods can also effectively compensate for various problems caused by groove wear. The first is to formulate a reasonable reduction adjustment and roll change and groove change system. According to the actual production, adjust and replace the material type of each rack in time to avoid the wear of the rolling groove of the rear rack due to the irregular material type of this rack. The second is the use of CNC lathes for roll processing. Shaped steel rolling adopts special-shaped pass, which is difficult to process. General turning processing is used. The lathe itself has low accuracy. Manual grinding of the turning tool causes the processing accuracy to be low. Therefore, the use of CNC machining can reduce the uneven wear of the rolling groove.

In conclusion

The rolling groove wear of section steel has its particularity. The sidewalls and arcs of the finishing pass have more wear than other parts, which fully reflects the unevenness of the groove wear.

Through the analysis of the actual groove wear data, factors such as the selection of roll material, pass design parameters, the relative sliding of the rolling piece and the roll, the processing accuracy of the roll, the formulation of the roll change and groove change system, etc., all cause uneven roll wear. the reason.

By analyzing the causes and taking corresponding countermeasures, such as laser surface strengthening technology, optimization of pass parameters, etc., it can effectively reduce the speed and unevenness of roll wear, stabilize production, and improve product quality.

Talking about How to Reduce the Loss of Narrow Strip Steel Roll Processing Procedure

 Abstract: Aiming at the problems in the repairing process of narrow-strip steel rollers, improvement opinions and solutions are proposed to increase its service life and reduce production costs.

As one of the most important processing parts and consumable parts of the rolling mill, the roll is subjected to fatigue stress and thermal cycling during use, which will cause serious fatigue wear and thermal fatigue cracks. In view of the high cost of rolls and high scrap rate, We currently choose the repair method of turning, grinding and reusing the rolls.

We have collected 50 rolls and made statistics on the processing volume. See the table below (unit:mm)

The calculation formula for the amount of loss (first roll diameter-last roll diameter) ÷ number of uses = loss

The calculated loss of each roller is shown in the following table:

Add up the 50 values to get 69.27mm and then calculate the average value: 69.27÷50=1.39mm. The 1.39mm obtained is to improve the average processing loss of the front roller.

 

1. Current problems and solutions

1.1 Imperfect turning process

(1) The turning efficiency is low, and the feed rate and speed have no clear limits on roughing and finishing. Solution: Clearly distinguish the values ​​of feed and speed in roughing and finishing.

(2) The machining loss in the turning process is large, and there is no segmented turning, so that the cylindricity of the processed roll does not meet the tolerance requirements, so repeated dressing.

Solution: adopt segment turning process

(3) The choice of turning tool material is unreasonable, and the strength of the 767 bit is insufficient, and it wears quickly during processing.

Solution: Use machine-clamped ceramic tools for turning.

1.2 The grinding process is not perfect.

(1) The grinding efficiency is low, and the feed rate and speed are not well matched during the machining process.

Solution: re-match the feed and speed of roughing and finishing.

(2) The processing loss in the grinding process is large, and there is no distinction between manual feed and automatic feed.

Solution: Change automatic feed to manual feed.

(3) The choice of the material of the grinding wheel is unreasonable, the material is soft, wears quickly, and the surface accuracy of the roll after processing does not meet the requirements.

Solution: Use microcrystalline corundum grinding wheel for grinding.

 

2. Roll repair process

2.1 Turning process

(1) Before processing the roll, use a dial indicator to check the coaxiality of the roll. During the processing, it should be controlled between 0 and 0.15mm, and should not exceed the tolerance.

(2) The spindle speed of the lathe during rough machining is 80~108r/min, and the cutting amount is 0.9mm/r. The lateral feed depends on the degree of roll wear, but cannot exceed 1mm. The spindle speed of the lathe remains unchanged during finishing. , The cutting amount is 0.2~0.4mm, and the lateral feed amount is 0.1~0.2mm

(3) The rough machining tool setting is located at the minimum wear diameter of the roll, and then the turning is carried out in the direction of both ends in sections.

(4) After the rough turning is completed, start the tool setting from one end of the roll, the cutting amount is 0.2~0.4mm, the lateral feed is 0.1~0.2mm, and the finishing is performed.

(5) After processing, use an outside micrometer to measure the two ends and middle diameter of the roll to determine whether the processing meets the technical requirements.

2.2 Tool and grinding wheel requirements. The turning tool is made of ceramic blades, and the material of the grinding wheel is microcrystalline corundum. The strength is 60.

The soft and bonding agent is ceramic.

 

3. Benefit calculation

3.1 Cost calculation of roll loss: Before the improvement, each roll is processed on average once and its process loss is 1.39mm, which is converted into weight: G1=πxDx(d/2)xLxp.

G1: weight loss in the previous process; D: average roll diameter; d: process loss; L: roll length;

3.14*275*0.695*500*7.87=2.36(kg)

After the improvement, each roll is processed once and its process loss is 0.79mm, and the converted weight is: G2=πxDx(d/2)xLxp. Calculate G2=1.34(kg)

After the improvement, the average weight saved is G1-G2=1.02 (kg). According to the current average roll price of 15 yuan/kg, each processing roll can save 15.3 yuan.

3.2 Grinding wheel consumption cost calculation: Before the improvement, each grinding wheel can grind an average of 80 rolls. The price of the grinding wheel is 529.92 yuan/piece, and the average consumption cost of the grinding wheel per roll is 6.624 yuan/piece. After the improvement, each grinding wheel can grind 220 rolls on average. The price of the grinding wheel is 581.2 yuan/piece, and the average consumption cost of the grinding wheel for each roll is 2.64 yuan/piece. After the improvement, the average cost of grinding wheel consumption per roll can be reduced by 3.98 yuan/piece.

3.3 Turning cost calculation: Before the improvement, each 767 head can process 17 rolls on average, and the average price is 30 yuan/piece. In addition, the cost of tool body and sticking tool is 15 yuan, and the average price of each turning tool is 45 yuan/piece. Finally, the average cost of turning tool consumption per roll is 2.65 yuan/piece.

After the improvement, each ceramic cutter head can process 64 sticks on average, 6 with an average price of 89.75 yuan/piece; other no additional fees. Finally, the average cost of turning tools per roll is calculated to be 1.40 yuan/piece. After the improvement, the average cost of turning tool consumption per roll can be reduced by 1.25 yuan/piece.

 

Finally, by adding up the savings and reduced costs of the above three items, the average cost savings per roll processing procedure is: 15.3+3.98+1.25=20.53 yuan/piece, calculated according to the average monthly processing of 794 rolls in the previous period, each month Compared with the pre-improvement, the cost saving is 16,300.82 yuan, and the annual intensive cost is 195,609.84 yuan. The actual operation results have proved that through the improvement and improvement of the roll turning and grinding process, the shortcomings and defects of the original process have been compensated, the best efficiency of the equipment has been brought into play, the repair cost has been reduced, the production efficiency has been improved, and the energy saving and consumption reduction of the main line is indirectly done. Made a contribution.

Cause analysis and improvement measures of cold rolling roll failure

 The main failure modes of Liugang Zhongjin's 950 mm six-high six-stand stainless steel narrow strip cold tandem roll are burst rolls (flaking), fractures, cracks, etc. This paper analyzes the causes of roll failure, and the results show that internal quality defects of the roll, unreasonable rolling process, and improper use of roll management are the main causes of roll failure. Through the implementation of measures such as strengthening the non-destructive inspection during the use of the rolls, optimizing the rolling process, strengthening the quality monitoring of raw materials, and standardizing the management of the use of rolls, the monthly strip breakage rate of stainless steel narrow strip cold tandem rolling has been reduced from 1.59% to 0.53%, and the number of roll failures has been reduced from 19 sticks/month is reduced to a stable control of 2~5 sticks/month, which effectively improves the production efficiency of the tandem mill and reduces production costs.

 

As a large-scale tool in steel rolling production, the performance and quality of the roll directly determine the output of the rolling mill and the quality of the product. Therefore, the use and management of rolls is very important in cold tandem rolling production. Liugang Zhongjin's 950mm six-high six-stand cold tandem cold rolling mill is the fourth domestic stainless steel narrow strip cold tandem rolling production line, and it is currently one of the high-level stainless steel narrow strip cold rolling mills in China. Since the start of production in July 2018, roll bursts (flaking), broken rolls, cracks and other accidents have frequently occurred during use. Therefore, analyzing the causes of roll failure and taking corresponding specific measures to improve the use and management of rolls and reduce the accident rate of rolls is of great significance for stabilizing production and reducing production costs.

The main forms of cold roll failure

The roll diameter of Liugang Zhongjin's 950mm six-high six-stand stainless steel narrow strip cold tandem rolling mill is small. The main technical parameters of the rolls are shown in Table 1. Since the start of production, roll scrap accidents have occurred frequently due to roll material, use management, rolling technology and other factors. Among them, the number of work rolls due to roll burst (flaking), roll breakage, and crack failure has been 54, 16, and 8, respectively. F1~F4 work rolls; 5 middle rolls are scrapped due to burst rolls; 7 support rolls are scrapped due to cracks and broken rolls. It can be seen that the main forms of cold roll failure are bursting (flaking), broken rolls, and cracks. The typical failure macroscopic morphology is shown in Figure 1.

Analysis on the Causes of Roll Failure

Roll material and shape design

Sampling and analysis of rolls due to spalling accidents, the source of fatigue, fatigue fracture zone, instantaneous fracture zone, and metallographic sample preparation position of the roll spalling are shown in Figure 2. After machining, the sample was corroded by a 4% nitric acid alcohol solution. The inclusions in the sample were observed and analyzed with a scanning electron microscope energy spectrometer. Granular Al2O3 inclusions and TiN precipitates were found inside the sample and near the cracks. The maximum size of the precipitates is 74.5 μm, and the maximum size of TiN precipitates is 18.3 μm. The micro morphology of sample inclusions, precipitates and internal cracks is shown in Figure 3.

It can be seen from Figure 3 that there are Al2O3 inclusions in the internal cracks of the roll, which may be caused by the roll manufacturer not using the electroslag remelting process during the smelting of the roll blank. When the roll is constantly subjected to alternating tensile and compressive stress during use on the machine, the separation of the inclusions and the matrix of the roll causes the discontinuity of the metal structure, resulting in internal cracks. As the crack extends, it will eventually cause the roll to burst (peel off) or even break the roll. In addition, the roll shape design and processing are improper, and the transition fillet design at the part where the cross-sectional size changes is unreasonable, resulting in stress concentration and roll breaking accidents. In the actual production process, the support rolls were broken due to the unreasonable design of the transition fillet when they were subjected to high rolling pressure. 

Rolling process and raw material quality

The cold roll is subjected to strong rolling pressure, bending moment, shear stress and other internal and external forces during rolling, resulting in cracks in the roll and peeling of the working surface, and the welds, inclusions, edge cracks, etc. of the rolled pieces can easily lead to instantaneous high temperatures, resulting in Accelerated wear of rolls, fatigue damage, etc. [3]. Therefore, the rolling process and the quality of raw materials have a significant impact on the service life of the roll.

(1) The degree of cold working hardening of stainless steel is relatively high. With the increase of deformation, the strength of the rolled piece increases rapidly, and the rolling force increases. When the total reduction is 67.5%, the tensile strength of the rolled piece is as high as 2000MPa. When the local stress of the roll exceeds the strength limit of the roll material, it will cause plastic deformation of the roll, and then cracks. With the action of the alternating tension and compression stress, the cracks propagate and cause the roll to peel off or break.

(2) The rolling tension control fluctuates or slips and causes the belt to break, so that the roll surface contacts the broken belt, and is subjected to the combined action of local overload thermal shock, temperature rise, and sliding friction, which causes the roll surface to produce different degrees of burns, resulting in the roll surface Flaking.

(3) Severe wedge shape or uneven hardness in the width direction of the raw material leads to the difference in the rolling force between the operating side of the rolling mill and the transmission side or the local overload thermal shock during the continuous rolling process, causing the pressure on the side with greater rolling force to exceed the strength limit of the roll material. The stress is concentrated, the edge of the roll is crushed and peeled, or the surface of the roll is peeled off due to the thermal shock of overload at the position with higher hardness.

(4) Raw material inclusions, edge cracks and poor weld quality lead to strip breakage during the continuous rolling process, and thermal shock causes the roll surface to stick to steel, cracks or peels.

Roll usage management

In the use and maintenance of cold rolling rolls, it is necessary to focus on several aspects such as roll change cycle, grinding amount, maintenance of defective rolls, stress recovery, and evaluation mechanism. In the early stage of production, the extensive management of roll use resulted in a high roll accident rate. The main reasons were:

(1) New rolls entering the factory, production accident rolls, etc., are used on the machine without flaw detection after being ground;

(2) The roll change cycle is unscientific, and the service cycle is too long. Under the action of the cycle stress, the work hardened layer due to the fatigue mechanism deepens, which increases the brittleness of the material, thereby increasing the possibility of cracks and spalling;

(3) There is no professional roll disassembly and assembly equipment, and the disassembly and assembly of the roll are completely completed by hand, which causes wear of the roll, chock, seal ring, etc., resulting in poor assembly quality and accuracy of the roll, and even burnout and jamming of the roll bearing, which causes the roll to twist. Break

(4) The shortage of spare parts for the rollers caused the lower rollers to be ground on the upper grinder without sufficient time for stress release. When the temperature of the roller surface is high, the grinding will cause burns and deteriorate the stress state of the roller surface, which is not conducive to restoring the normal stress. , In severe cases, it will cause cracks or peeling when grinding on the grinder;

(5) Insufficient staff training has led to unclear roll grinding, recording, inspection and responsibility for accidents. It is necessary to formulate defective roller handling specifications and inspection standards to prevent defective rollers from being used on the machine.

Improvement measures and effects

Increase the intensity of roll inspection and inspection

Due to the small roll diameter, traditional large roll companies are not very enthusiastic about production. The rolls currently in use are produced by smaller roll manufacturers in China, and the quality of the rolls fluctuates greatly. During the initial stage of operation of the stainless steel narrow strip cold tandem mill, there were many accidents of spalling, roll breaking and cracking caused by roll quality problems. In order to avoid the use of unqualified rolls on the machine, it is required to conduct a flaw detection inspection on each new roll and the accident roll before it is placed on the machine. The rolls that are normally planned to be replaced are randomly checked. Using the existing detection method combining ultrasonic flaw detection and magnetic particle flaw detection, using the good penetration and directivity of ultrasonic waves, it can locate, quantify and qualitate the internal defects of the roll. At the same time, magnetic particle flaw detection assists in detecting the surface quality of the roll. The eddy current flaw detection of the grinding machine found that the rolls with defects on the surface and inside were refused to be used on the machine, and the roll non-destructive testing management system was formulated.

Strengthen the quality control of raw materials

At this stage, the quality of the upstream raw materials of stainless steel narrow strip fluctuates greatly. Quality problems such as raw material edge cracks, inclusions, and peeling cause frequent strip breakage in cold continuous rolling, resulting in roll burning, steel sticking, cracks, and spalling. Therefore, it is necessary to strengthen raw material quality monitoring. The strip steel with the risk of strip breakage must be dealt with according to corresponding measures; Optimize the rolling process, reasonably allocate the reduction of each frame of F1-F6, and control the total reduction rate between 50% and 70%; reduce the hardness of the work roll Reduce from 90~93HSD to 86~90HSD; adjust the rolling force difference between the operating side and the transmission side of the rolling mill and the tension fluctuation range between the stands to prevent unilateral waves in the shape control, prevent the strip from breaking, causing the roll to stick to the steel, Cracks, even peeling.

Standardize roll use management system

Formulate a standardized roll management system:

(1) A reasonable roll change cycle can not only give full play to the use performance of the rolls, but also ensure the rolling quality;

(2) In order to ensure the release of the residual stress of the lower rolls and restore the normal stress, a certain number of spare parts for the rolls must be guaranteed. Normally, the rolls of the off-machine cannot be arranged for grinding immediately. The work rolls, intermediate rolls, and backup rolls are required to be off-machine for 12, 24, and 48 hours before being ground;

(3) Increase the roll disassembly and assembly platform to improve the quality and accuracy of roll disassembly and assembly; (4) Establish a roll evaluation mechanism. Number the rolls, quickly identify them, and manage roll files to track the usage of each roll.

Improved effect

By strengthening the non-destructive inspection during the use of the rolls, optimizing the rolling process, strengthening the quality monitoring of the raw materials, and standardizing the use and management of the rolls, the monthly stripping rate of 950 mm cold tandem rolling has been reduced from the highest 1.59% to 0.53%, and the monthly roll failure count has been reduced from the highest 19 The support is reduced to a stable control at 2~5, which effectively improves the production efficiency of the continuous rolling mill and reduces the production cost.

Concluding remarks

Spalling, broken rolls, and cracks are the main forms of roll failure. During roll use, non-destructive flaw detection can be strengthened, rolling process and raw material quality monitoring are optimized, roll use management is standardized, and rolls with quality problems can be avoided from unintended use and production can be reduced. Accidents, reduce the number of roll failures, stabilize production, and reduce production costs.

Automatic sampling during steelmaking process

Cause Analysis and Improvement of Burst of Cold Rolling Roll

Abstract: This article analyzes the phenomenon of roll cracks and roll bursts in the cold rolling production process, and combines actual production conditions to propose measures to avoid roll cracks and bursts.

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.