Analysis on the Causes of Surface Spalling of Work Rolls

Cold rolling rolls are important large parts of cold tandem mills. Their manufacturing process is relatively complex and the working environment is harsh. They bear the effects of friction, thermal stress, impact and other stresses. During use, they may break, peel, wear, etc., and then fail. , Increase the cost of consumables and affect production, causing economic losses. A company's stainless steel production line suddenly peeled off the surface of the cold tandem work roll during the normal rolling production process, causing a breakdown and shutdown, which seriously affected the normal production. This paper takes the peeling sample of the cold tandem rolling work roll as the analysis object, and analyzes and explores the reasons for the peeling of the roll surface through physical and chemical testing methods such as macroscopic fracture, spectral composition determination, hardness, metallographic structure, and scanning electron microscope, combined with daily use conditions.

Physical and chemical test results 

Fracture observation of spalling block

The macroscopic morphology analysis of the roll peeling block (see Figure 1). There are typical fatigue fracture characteristics on the spalling block. The obvious shell pattern line can be observed in the central circle area, which is an important feature of the fatigue growth zone; the center of the shell pattern line is the fatigue source, which is the initiation area of the fatigue crack. Fatigue can be seen The source is located inside the roll rather than on the surface; the outer ring area occupies the largest area, and the macroscopic morphology is radial, which is the instantaneous fracture zone. In the failure cases of structural materials and mechanical parts, fatigue failure is different from static load failure. Most of them occur without warning and unpredictability. There is no obvious sign before failure, and the damage is serious.

Observing the fatigue source area with scanning electron microscope, it is found that there are granular inclusions with a diameter of about 50μm. The EDS results show that they contain Ca, K, O, Mg, Al, Si and other elements (see Figure 2), which are large-particle oxide mixed inclusions. Things.

Chemical composition A sample is cut from the exfoliated block for spectral composition analysis. The material of the roll is 8Cr3NiMoV. The test results are shown in Table 1. Except for the Cr element which is slightly below the lower limit, the other components are all within the standard GB/T1299-2014 range. Using Rockwell hardness tester to test the hardness of the peeling block, the hardness (HRC) value reaches 64.5, and the positions on the sample are relatively uniform, which meets the requirements of the standard.

Metallographic observation 

A wire cutting machine was used to intercept the metallographic sample at the fatigue source. After measurement, the fatigue source was about 8mm away from the surface of the roll; after sample preparation and polishing, the ZEISSImager.A1m metallurgical microscope, ZEISSEVO18 scanning electron microscope and other instruments were used for observation and analysis. It is found that there are many root-shaped cracks in the spalling block (see Figure 3). The cracks originate from the location of the internal fatigue source and extend to the surface of the roll.

A comprehensive observation of the sample reveals that there are more massive inclusions in the peeled block (see Figure 4a), with a size of 90 μm, and the inclusions are seriously exceeding the standard. Using scanning electron microscope energy spectrum analysis (see Figure 4b), the bulky large-particle inclusions are mainly O, Al, and a small amount of Mg, S, Ca, Mn, and Fe. It can be inferred that they are mixed inclusions with Al2O3 as the main component. It may be caused by inadequate control of the smelting process in the production of rolls.

Analysis and discussion 

Cracking process

From the macroscopic analysis, it can be inferred that the failure of roll spalling originates from internal fatigue sources. Then, under continuous cyclic stress during work, the cracks gradually propagate to form shell lines; fatigue cracks propagate to a certain extent, resulting in insufficient strength and unable to withstand rolling. The external force during the manufacturing process will eventually form a terminating zone and the surface of the roll will peel off. During the operation of the roll, due to the load of the rolling mill and the local squeezing of the roll at the contact point, the maximum combined shear stress is located in a small area below the surface of the roll. The preparation process of the roll before manufacturing and use will produce residual stress. At the same time, although the cold rolling processing temperature is low, the temperature of the roll and the strip steel will also increase under the action of friction to produce thermal stress. If strip breakage, tail flicking, overlap, slippage, etc. occur during the rolling process, the surface of the roll will be subject to local overload heat and impact stress. Because non-metallic inclusions exist in steel in the form of mechanical mixtures, and their performance is very different from steel, they destroy the uniformity and continuity of the steel matrix, and cause stress concentration at the place, which becomes a source of fatigue. In addition, during the heating process, non-metallic inclusions

The linear expansion coefficients of the object and the matrix are different, and an additional stress field is generated in the matrix near the inclusions. Under such complex stress conditions, if there are non-metallic inclusions in the surface layer, especially brittle inclusions, they will peel off from the matrix at the two extremes of the maximum stress of the spherical inclusions, forming primary microcracks; Before the rapid expansion, the junction between the steel matrix and the inclusions gradually separated from the matrix and connected into a crack channel. With the increase of the number of stress cycles, the microcracks gradually expand outward along the shell of the ball torn the matrix. When the overall size of the crack exceeds the critical size that the roll can withstand, the fatigue crack enters the instability growth stage, and the surface of the roll is finally caused by the occurrence of fatigue cracks. It breaks instantly and peels off. The surface peeling of the roll has undergone the processes of crack initiation → crack propagation → peeling caused by inclusions.

The influence of inclusions on fatigue performance

The influence of inclusions on the fatigue life of the workpiece is related to the nature, size, quantity and distribution of the inclusions. Generally speaking, hard and brittle blocky or spherical inclusions that have poor connection with the matrix and do not deform, such as TiN and Al2O3, are more harmful than ductile and elongated inclusions. When the number of inclusions is large, when they are concentrated and distributed, or when they are on the surface of the part or in the high stress area, the fatigue life is most severely affected. At the same time, the influence of inclusions on fatigue performance also depends on the structure and properties of the matrix. Experiments indicate that the fatigue strength of mild steel has a smaller relationship with inclusions. As the strength of steel increases, the harmful effects of inclusions become more and more. serious. In metal materials with high hardness and high strength, the influence of inclusions on fatigue strength becomes a more prominent problem.

According to relevant research data, fatigue life is very sensitive to the size of inclusions. Reducing the size of inclusions can significantly increase fatigue life. For high-hardness and high-strength workpieces, the critical size of surface inclusions is 8-10μm, and it decreases as the hardness increases, and increases as the depth increases. Inclusions smaller than the critical size can avoid the fatigue fracture caused by the inclusions, and the fatigue performance will be better; when the inclusions are larger than the critical size, as the size of the inclusions increases, the fatigue strength and fatigue life of the steel decrease sharply. According to the literature, for high-strength steel, if the size of the inclusions is reduced by 1/3, the fatigue life will be extended by 10 times, and if the size of the inclusions is reduced by about half, the fatigue life will be extended by 100 times. At the same time, if the size of the inclusions is reduced by half, the fatigue strength can be increased by 1.12 to 1.15 times.

To sum up, the roll is under the action of complex stress during use, which produces fatigue crack sources at large-scale inclusions. With the continuous change of alternating stress, fatigue cracks propagate and generate secondary cracks, which are formed on the surface of the fracture. Fatigue bands and radial bands. When the overall size of the crack exceeds the critical size, the fatigue crack enters the instability growth stage, and the roll eventually peels off due to instantaneous fracture.

Conclusion 

(1) The surface spalling of the cold tandem rolling work roll is the fatigue fracture caused by the large-size Al2O3 brittle and hard inclusions near the surface.

(2) Strengthen the inspection of rolls. Ultrasonic testing can detect internal defects, and magnetic powder and eddy current can detect surface defects; use different methods or combinations according to the situation to ensure the quality of the rolls.

(3) Develop a scientific and reasonable roll use and maintenance system to ensure cooling and lubrication during use and prevent overheating.

Research on Operation and Maintenance of Cold Roll

 Abstract: Roller is the key contact technology equipment for strip production. In view of the problems in the process of cold rollprocessing, the surface quality is improved in the production process from the aspects of roll grinding, non- destructive testing of rolls, andthe use of rolls, and the production cost is reduced to ensure continuous and efficient operation. Non-destructive testing of rolls is of greatsignificance for the rolling of cold rolled sheets, especially ultrasonic testing. Rolling accidents are reduced during the on-line operation ofthe rolls. According to the  attenuation of the roll roughness, the schedule is reasonable, the chrome-plating quality of the chrome-platedrolls and the edge chamfering of the support rolls are optimized to reduce the roll consumption.

0 Preface

With the continuous upgrading of cold-rolled products, cold-rolled products are not only for ordinary building materials, but are also moving towards high-end automotive and home appliance plates. The variety of cold-rolled products has achieved full coverage from low-carbon and ultra-low-carbon mild steel series to high-strength and ultra-high-strength hard steel series. The surface quality of the strip needs to further meet the O5 automotive exterior surface requirements. The rolls are in direct contact with the strip steel, and the surface appearance of the work roll is equivalent to the "mold", which is the key to determining the surface quality of the strip steel. Higher requirements are put forward for cold-rolling work rolls or intermediate rolls, with higher hardness and higher wear resistance. At the same time, new requirements are put forward for the metallurgical quality and microstructure of the working layer of the support rolls, with stronger toughness and Anti-stripping performance. Roll grinding and inspection

And improper use will cause a sharp increase in roll consumption. This paper studies the rolls from the three aspects of roll processing, testing and use, in order to improve the service life of the rolls, improve the quality of the strip, and provide a reference for enterprises to reduce the consumption of the rolls and improve the quality of the strip.

1 Roll processing

During the working process, the roll will bear high rolling load, and at the same time, there will be frequent bending, torsion and friction between the rolls. After a certain period of use, the surface needs to be ground to remove surface pits, roll marks, cracks and other defects. , The fatigue layer is gradually removed while repairing the roller curve. The roll material is a polymer composed of iron-based solid solution and alloy carbides. Currently, Cr5 rolls are generally used, with a single grinding volume of 0.2-0.3mm. If the roll processing cannot be qualified once, the grinding two will also reach 0.1 mm or more.

In addition to the mechanical equipment factors of the grinder, the most critical media and consumables for roll grinding are metal cutting fluid and grinding wheel, and the improvement of surface quality also puts forward higher requirements for metal cutting fluid and grinding wheel. Metal cutting fluid needs

It has good chip settling properties, excellent cooling and cleaning properties, and reduces scratches on the roller surface. Select the appropriate grinding wheel model and grinding process parameters to ensure that roll shape problems, surface chatter marks, transverse thread lines, surface roughness, etc. cannot occur during the grinding process. For hardened steel rolls, corundum grinding wheels are generally used, and the corundum grinding wheels that match them can be selected according to the material of the rolls to achieve high surface quality and grinding accuracy. Problems with roll shape are usually mainly caused by too soft grinding wheels. Vibration defects on the roll surface will occur if the grinding wheel is too hard or the feed rate and lateral movement speed are too high. It is necessary to reduce the linear speed of the grinding wheel, reduce the amount of feed, reduce the lateral movement speed, and increase the rotation speed of the roll. The horizontal thread line is also called the cutting pattern, which will appear when the grinding wheel is not corrected well or the feed rate or lateral movement speed is too high. When the grinding wheel is too soft, the filter of the coolant fails, and the abrasive particles float in the coolant, it will cause the grinding point defects on the surface of the roll. It is necessary to check the filtration of the coolant, increase the speed of the grinding wheel, reduce the speed of the roll, and reduce the amount of feed . The grinding wheel is too hard, the self-sharpening property is poor, the passivated sand particles are not easy to fall off, and the grinding roller surface is prone to fuzzing. It is necessary to increase the roller speed and reduce the linear speed of the grinding wheel.

The grinding feed rate of the roll and the cooling of the cutting fluid not only affect the surface condition of the roll, but too large feed rate or insufficient cooling will also cause micro-cracks on the surface of the roll. The micro-cracks are small and dense, and they are not damaged. Testing such as eddy current and ultrasonic testing requires the sensitivity to be adjusted to a high level to detect. Ultrasonic and magnetic particle inspection results of micro cracks on the surface of the burnt roller. If this kind of grinding micro-cracks is not found, the use of the machine will cause serious accidents of peeling and roll bursting.

In order to achieve good deep drawability and high coating adhesion, the surface of cold-rolled strip is usually pitted, and the work roll needs to be textured. At present, the roll texturing method used in the production of high-quality strips mainly adopts EDM texturing. The topography parameters of EDM texturing are surface roughness Ra, peak density Pc value, etc., which can be adjusted by pulse voltage, point spacing, current Intensity, intermittent time, etc. are controlled. Reducing the current or pulse width can reduce the Ra value and increase the PC value. The uniformity of the texturing and the point defects of the texturing are related to the quality of the carbon powder and the electrode. Uneven discharge and carbon powder adhesion will cause texturing quality problems, leading to repeated grinding and texturing, resulting in increased costs. At the same time, watermarks often appear in the texturing process. The watermarks are wavy and curved, and there is a certain color difference on both sides, which is more obvious after chrome plating, which has a serious impact on rolled high-end surface products.

2 Roll inspection

Non-destructive testing is required before rolls are put on the machine. The usual methods used in daily production include ultrasonic testing, eddy current testing, magnetic particle testing and penetrant testing. Eddy current testing has the advantages of fast detection speed and can detect soft spot defects, but it can only target surface opening defects, and there are many factors affected by interference. Excessive sensitivity can easily cause messy signals. Penetration testing is only effective when dealing with locally large opening defects. The use of ultrasonic and magnetic particle detection is more effective in ensuring the normal use of the roll.

Ultrasonic surface wave is more convenient to use the bottom wave method, that is, the probe is placed at a position of 1000mm in the middle of the roll body, the diamond reflection wave at the end of the roll body is adjusted to 20% high, and then the gain is 32dB. When the roll is in good condition, the surface of the roll body will not cause abnormal wave reflection regardless of the circumferential and axial detection of the roll body. However, due to the restriction of the smelting process, there are sometimes tiny pores and loose defects in the outer ring of the roll body. After forging, The elongated strips are called hair lines, which are randomly distributed along the length of the roller body, and the length is mostly 0.05-3mm, which causes abnormal reflections in surface wave detection. These small hairline defects are generally within 1mm in length, and will not affect the quality of the product during use. However, it is difficult to distinguish the small cracks produced by the roll on-line rolling from the small cracks during the ultrasonic inspection. In this case, magnetic particle inspection and microscopic observation are needed. The edges of the defects are generally flat and are generally caused by roll manufacturing, and those with sharp edges are generally crack defects caused by rolling. Surface crack defects can cause strip-like peeling on the roll surface, effectively detecting the residual cracks on the roll surface that have not been cleaned, and avoiding the rolls from being damaged and running on the line. Magnetic particle and ultrasonic inspection results of hairline and magnified image of residual cracks.

The direct detection sensitivity of ultrasonic flaw detection is determined according to the AVG method to determine the equivalent diameter of the flaw, and 20% of the first bottom wave reflection is not full of the screen height as the reference height, and then the sensitivity increment of the flaw detection part is calculated as follows to adjust the gain.

Calculation formula n=20lg (λD/2π), n———the value of sensitivity increment (dB); D———the thickness of the workpiece (unit: mm);

λ———When f=2.5MHz, the wavelength of ultrasonic wave in steel (usually λ=2.36mm).

After the defect signal is found, the gain of the instrument is used to divide the thickness of the detected workpiece according to the principle of 12, 7, 5, 4, 3, 3, 2, 2, 2, and the corresponding gain value is decremented according to the defect position. As a benchmark, raise or lower the height of the defect wave to 20% of the full scale of the screen. Generally, the outer surface layer of the work roll and the intermediate roll with a depth of 90mm from the roll surface is not allowed to have defective echoes, and the middle area is not allowed to have defects exceeding Φ2+15dB. Although the abnormal wave defect near the center of the work roll caused a certain drop in the bottom wave, the evaluation result did not exceed the standard. The abnormal wave comes from the superimposed equivalent of the loose center, and the roll is used on-line without any abnormality.

The use of the support roller, the stress concentration position during use is 5-10mm depth. It is difficult to find defects in this depth area through ultrasonic surface wave inspection after offline. The defect formation position is relatively shallow when detected by a straight probe. If a certain scale is not formed, the inspection will be more difficult. In this case, improve the sensitivity of surface waves, screen grass waves, and focus on direct ultrasonic detection of beam positions that appear in clusters. Ultrasonic detection and internal and external conditions of damage under the roll skin. In the follow-up use, it is necessary to increase the intensity of detection and tracking of accident rolls to prevent the continuous expansion of cracks and cause the entire roll to be scrapped.

3 Roll use

In the case of large reduction in acid tandem rolling, the roll body hardness of the work roll of the rolling mill must be absolutely guaranteed, otherwise it will cause product thickness fluctuations and poor plate shape. The pursuit of roll height and hardness will lead to poor roll anti-accident ability, which puts forward higher requirements for a low production line operation accident rate. A more serious broken belt accident will directly cause the work roll worth 100,000 yuan to be scrapped. Downstream users, such as automotive appliances, put forward higher requirements for the shape of cold-rolled products, and at the same time, the range of surface roughness requirements for automotive steel is very narrow. In order to obtain the surface roughness and surface quality of the strip steel that meet the requirements of users, it is necessary to arrange the production plan reasonably according to the attenuation of the roll roughness, and extend the life cycle of the roll as long as possible to reduce the frequency of roll change. Reduce roll consumption.

The relationship between the roller change cycle of the support roller and the structure and specifications of the product is relatively small. In addition to the internal damage caused by the accident, the abnormal consumption of the roll is at the position where the shoulder chamfer of the support roll is the most stressed, and the chamfer at the end of the roll body is in a straight line, it is easy to form small micro cracks, and the crack depth can reach 2 -3mm. Grinding and removal will cause a lot of roll consumption. If it is not handled in time in subsequent use, it will cause the side of the roll to peel off. Changing to arc chamfer can effectively reduce the stress concentration at this position. Quality problems such as the inclusion of the roll itself will also cause the fatigue resistance of the chamfered part to decrease. Even if the chamfering curve is optimized, the inclusion defects in this area will cause the initiation and propagation of cracks and affect the life of the roll.

In the rolling process, the chrome-plated roll can reduce the coefficient of friction, and the amount of iron powder generated during the roll gap friction process is reduced, which greatly improves the cleanliness of the strip after rolling. Improve the average reflectivity of the surface of the strip. The pressure in the subsequent continuous annealing and galvanizing cleaning section is reduced, which is beneficial to improve the quality of the board surface. Table 1 shows the comparison of the effect of using chrome-plated rolls in Tangshan Steel's cold rolling mill. However, the quality of the coating is very high, and it is necessary to ensure that it cannot fall off under a large rolling load. Once the coating falls off, it will directly cause surface defects of the product. The normal thickness of the chrome plating layer is about 6μm, and the microstructure is in the form of a net.

4 Conclusion

① The manufacturing control of the roll itself and the optimization of subsequent grinding and texturing processing parameters are the key to ensuring the quality of the roll surface. The pass rate of roll grinding increases by 5%, and the overall service life of the roll is extended by more than 2.5%. Promotion and cost reduction play an important role.

②The defect prevention of non-destructive testing and the stable operation of on-line testing equipment basically avoided the loss of roll peeling from 1-2 cases per year.

③The attenuation law of roll roughness, the quality of roll chrome plating and the design of the operating cycle can increase the output of products with high-quality surface requirements by 80%, and basically reduce the single-roll production schedule of 3-5 rolls in each roll cycle. The brought products are an important factor in improving product quality and reducing costs.

Effective way to reduce roll consumption

 1. Overview

The rolls are in direct contact with the rolled steel strip and are an important tool for hot rolling production. Reducing roll energy consumption effectively reduces production costs and increases benefits. At the same time, it can effectively increase the mill operating rate and ensure the surface quality of hot-rolled products. In the production of hot-rolled strip steel, the consumption of rolls is particularly huge. Therefore, finding out measures to reduce roll consumption and increasing the life of the rolls is of great significance to the production of hot-rolled strip steel.

2. Optimize the roll change cycle

2.1 Roughing work roll

The roll change cycle of the original rough rolling is 2.5wt～3.8wt. On this basis, the roll change cycle is gradually extended, and the wear condition, strip shape and surface quality of each roll are compared. The roll change cycle is now stable at 3.8wt～4.5wt. It does not affect the strip shape and surface quality, but also prolongs the service life of the roll.

2.2 Finishing back-up roll

The F1～F7 rolls have different rotation speeds and a large difference, so the wear degree of the back-up rolls is also different. The original one-knife-cutting roll change cycle of 10wt to 15wt is not reasonable.

Since the adoption of the segmented roll change, not only has the rolling volume of the roll increased by about 14.94%, thereby prolonging the service life of the roll, but also according to statistical data, the wave shape when rolling thin gauges at the end of the roll use has also been reduced.

 

3. Correct handling method of rolling accident

Serious rolling accidents directly increase the value of roll consumption, and whether the accident roll can be handled correctly after the accident is the key to reducing roll consumption.

3.1 Card steel accident

After the steel jam occurs, immediately stop the cooling water and lift the upper roll to avoid the upper roll contacting the strip steel; after the strip is removed, change the roll and allow the roll to cool slowly. If the roll cannot be changed immediately, check that the crack is not serious, and the work roll can enter normal rolling without cooling water idling for 5-10 minutes to make the temperature spread evenly.

3.2 Sticking steel accident

First remove the sticky steel. Afterwards, a detailed inspection should be carried out on the steel-bonded area, including hardness, cracks, flaw detection, etc. The affected area of the steel-bonded steel must be removed to ensure that the roller surface has uniform hardness and no cracks.

 

4. Innovative roll defect repairing countermeasures

From August 2015 to April 2016, unplanned roll changes statistics show that accidental rolls accounted for 69% of the total roll changes. Therefore, after a rolling accident occurs, timely and correct handling of the rolls can minimize unnecessary roll damage, extend the service life of the rolls, and reduce roll consumption.

The following briefly introduces the treatment methods of several common accident rolls:

4.1 Thermal cracking

Hot cracks are mostly caused by the chilling and heating of the rolls after the stuck or piled steel accidents.

Treatment method: R1 and F1-F4 rolls can be used on-line with hot cracked rolls under the premise of ensuring smooth rolling, and the cracks are closed (closed cracks do not extend, but prolong with the use time), cracks Will gradually eliminate.

4.2 Roll sticking to steel

The roll sticks to steel, and the roll sticks to strip steel due to tail-flicking or steel pile accidents.

Treatment method: the roll body strip steel sticking is serious, the area is large, and the sticking strip steel needs to be turned off first when the thickness is more than 3mm. After the adhered iron car is dropped, check the surface of the roller body. If there are cracks and flaking (flesh), determine the depth of cut according to the degree of damage to ensure that defects are eliminated without increasing excessive roller consumption.

4.3 Peeling off the edge of the work roll body

The edge of the work roll body is peeled off, which is caused by stress concentration at the roll shoulder.

Treatment method: Since the shoulder of the roll does not directly touch the steel plate, it can remove the cracked and peeled part, grind the chamfer, and adjust the chamfering method and angle of the supporting roll in contact with it to improve the stress distribution of the roll body.

4.4 Defects of backup roll

The back-up roll is defective. The back-up roll does not directly contact the strip steel and its hardness is lower than that of the work roll, but it is expensive, so it is not appropriate to use conventional grinding methods.

Treatment method: When the damage is on the edge of the roll, appropriately increase the chamfer length of the end when grinding. The remaining defects are treated with an angle grinder and the surroundings are smooth and excessive to prevent the peeling defects from spreading further. When the damage is in the middle of the roll, it should be ground as usual and then treated with an angle grinder as well, and the defect should be smooth and excessively round.

 

5. Adjust the roll profile

According to the actual production situation, the F1-F4 rolls are changed from concave rolls to HVC roll types. And increase the negative crown of F5-F7 rolls, thereby improving the stress distribution on the roll surface. It can achieve better control of plate shape and reduce roll consumption.

 

6. Other ways to reduce roll consumption

① Organize exercises to deal with various rolling accidents, increase the speed of emergency response, and minimize the damage to the rolls caused by the accident;

② Formulate reasonable roll inspection standards to ensure roll quality while reducing unnecessary losses;

③Choose excellent roll manufacturers and customize high-quality rolls with customized material and hardness that meet the production characteristics of the heavy steel hot-rolled sheet plant;

④ After the production is stable, high-speed steel rolls are selected for the front work rolls of finishing rolling to further reduce roll consumption.

 

7. Conclusion

① The adjusted HVC roll profile and negative crown roll not only improve the surface stress distribution of the roll, but also strengthen the crown control ability of the strip, which is beneficial to the guarantee of product quality.

②Adjust the roll change cycle, the response to the rolling accident and the correct handling of the accident roll. While ensuring the quality of the product, it increases the rolling volume of a single roll and prolongs the service life of the roll.

③The countermeasures for accident rolls reduce unnecessary grinding of rolls, thereby reducing the labor intensity of employees.

rolling mill

In-situ observation of surface oxidation of roll

 During the use of the roll, due to the surface contact between the roll and the hot billet, an oxide film is formed on the surface under the action of heat. The formation and peeling of the oxide film have a significant impact on the consumption of the hot roll and the quality of the hot rolled product. In situ observations were made on the surface oxidation of hot rolling rolls of three materials: high chromium cast iron, high chromium cast steel, and high-speed steel rolls.

In-situ observation of surface oxidation during continuous heating (a→l arranged from left to right, from top to bottom)

(a) High chromium cast iron 250℃; (b) High chromium cast iron 400℃; (c) High chromium cast iron 500℃; (d) High chromium cast iron 650℃;

(e) High chromium cast steel 250℃; (f) High chromium cast steel 400℃; (g) High chromium cast steel 500℃; (h) High chromium cast steel 650℃;

(i) High-speed steel 250℃; (j) High-speed steel 400℃; (k) High-speed steel 500℃; (l) High-speed steel 650℃.

In situ observation of surface oxidation at 650℃ isothermal process (a→l arranged from left to right, from top to bottom)

(a) High chromium cast iron 0min; (b) High chromium cast iron 10min; (c) High chromium cast iron 30min; (d) High chromium cast iron 60min;

(e) High chromium cast steel 0min; (f) High chromium cast steel 10min; (g) High chromium cast steel 30min; (h) High chromium cast steel 60min;

(i)High-speed steel 0min; (j) High-speed steel 10min; (k) High-speed steel 30min; (l) High-speed steel 60min.

The influence of the number of heating and water cooling cycles on the oxidation morphology of the sample surface (a→l arranged from left to right, from top to bottom)

(a) High chromium cast iron 10 times; (b) High chromium cast iron 30 times; (c) High chromium cast iron 50 times; (d) High chromium cast iron 100 times;

(e) High chromium cast steel 10 times; (f) High chromium cast steel 30 times; (g) High chromium cast steel 50 times; (h) High chromium cast steel 100 times;

(i) High-speed steel 10 times; (j) High-speed steel 30 times; (k) High-speed steel 50 times; (l) High-speed steel 100 times.

Conclusion

In the continuous heating process, the oxide film formation temperature of high chromium cast iron and high chromium cast steel is lower, about 300 ℃, but the oxide film is dense and uniform, which inhibits the growth of the oxide film in the high temperature and isothermal process; while the high speed steel oxide film is formed The temperature of the oxide film is higher, and the temperature at which the oxide film is formed quickly is about 480°C, but the oxide film has poor uniformity and has a faster growth rate.

At high temperatures, due to the poor compactness of the oxide film formed around the granular MC-type carbides in the high-speed steel, the formation of a uniform oxide film is split, resulting in the oxidation resistance of high-speed steel lower than that of high-chromium cast iron and high-chromium cast steel. Therefore, compared with high-chromium cast iron or high-chromium cast steel rolls, high-speed steel rolls must achieve better results during use, and require higher production processes and equipment.

rolling mill

Intelligent palletizing system, development of magnesia carbon brick aut...

Analysis and Prevention of Staggered Rolls in Short-stress Rolling Mill

1.Introduction

Due to the axial movement of the rolls or other reasons, the pass centers of the upper and lower rolls are not coincident, so the rolled pieces of the correct shape and size cannot be rolled, which affects the quality of the product. This defect is called the wrong roll. Recently, under full-load production conditions, the number of mis-roll defects caused by short-stress rolling mills has increased. This article analyzes the causes of the wrong roller to propose solutions.

2. The cause of the wrong roller

2.1 Reasons for the axial movement of the rolls

Check and analyze the short-stress rolling mills that are offline due to axial movement. The reasons for the axial movement are as follows:

(1) The thrust bearing is damaged, resulting in a large amount of axial movement.

(2) Bearing end cover, internal thread end cover, bearing seat, internal thread through cover end surface or thread is worn, and the clearance is too large, and axial movement will also occur.

 

(3) The tension rod, the pressure-bearing flange, and the pressure-bearing ball pad are worn out, and the elastic damping body fails, etc., which will also cause axial movement.

 

(4) The roll locking device is not tightened, and the axial adjustment device is too loose or too tight, which will also cause axial movement.

 

(5) During roll turning, the gap between the upper and lower roll holes is inconsistent, resulting in misaligned rolls (not axial movement).

 

2.2 Wrong roll caused by steel rolling operation

(1) The axial adjustment mechanism of the short-stress rolling mill has two types of layouts: left and right. The worm gear and worm spare parts of the two arrangements are the same and can be interchanged. However, the direction of axial movement of the adjusting roller is completely opposite (see Figure 1). When the adjusting worm is at the left end of the bearing housing, turn the adjusting worm clockwise, the roller rotates clockwise, and the upper roller moves along the non-adjusting end. When the adjusting worm is at the right end of the bearing seat, turn the adjusting worm counterclockwise, the roller rotates clockwise, and the upper roller moves along the non-adjusting end. Limited by the number of rolling mills, in the state of full-load production, the short-stress rolling mills are used in various lines, and the support operators come from each production line. If the operators do not pay attention to the position of the worm, adjust the direction of the axial movement of the rolling mill. Past experience may make adjustments in the wrong direction. At this time, the more you adjust, the greater the amount of mis-roller. Eventually, the adjustment mechanism will be crushed, and the adjustment will not be possible, and the roller will be forced to change.

After the roll pass is adjusted in the axial direction, if the operator does not lock the set screw, the wrong roll will occur after a period of production.

3. Measures to solve the wrong roller

(1) Re-preload the rolling bearing to eliminate the gap caused by the reduction of the pre-tightening force caused by the wear of the rolling bearing and related parts, and realize the axial pre-tightening.

 

(2) Give a certain axial load when installing the thrust bearing to prevent and eliminate the axial movement caused by the same period vibration.

 

(3) On-site steel rolling operators must fully grasp the principle and adjustment operation of the axial adjustment mechanism.

 

4. Conclusion

The operation of short-stress rolling mill equipment is closely related to the quality of equipment maintenance and repair. The correct rolling operation method is of great significance for improving work efficiency and product quality.

Rolling Mill

Intelligent palletizing system helps the innovation and development of magnesia carbon brick automated production line!

 

• Robot intelligent palletizing system

The robot intelligent palletizing system can be equipped with a shaped refractory electric screw press and a fully automatic hydraulic press to realize intelligent appearance cleaning, quality inspection, brick marking, standard stacking, and screening of unqualified products. Meet the needs of enterprises for automated production of shaped refractories.