Analysis on the causes of breakage of work rolls in plate and strip mills

Abstract: This article describes the fracture problem of the strip work roll, describes the fracture state, analyzes the causes of fracture, explains the mechanism and function of fracture for various reasons, and proposes measures to avoid roll fracture.

Keywords:work rolls; fracture; residual stress; structural defects

Preface

Roll fracture is the most serious form of roll damage. It will not only cause the loss of rolls and rolled materials, but may also affect the safety of equipment and people. It will take a long time to deal with the accident and affect the operation rate of the rolling mill.

Breakage of the strip work roll can occur at the roll body, roll neck, and transmission end shaft head. There are usually two types of causes for roll fractures: one is caused by the roll material or manufacturing quality, which is caused by inherent defects in the roll itself, such as excessive residual stress after manufacturing, casting defects or improper heat treatment causing abnormal structures, internal inclusions, etc. The other type belongs to the rolling process conditions and usage conditions, that is, the stress is increased due to external reasons, such as severe thermal cracking on the roll surface, overloading and poor cooling conditions. In many cases, roll breakage is caused by a combination of factors.

Main fracture modes and causes

Residual stress and thermal shock fracture

At the beginning of rolling, the roll is in contact with the high-temperature rolling stock. As the rolling process proceeds, the temperature gradually rises. During this process, the surface temperature of the roll is high, but the core is still at a low temperature, and the temperature is distributed along the radial direction. As shown in Figure 1.

Figure 1 Roll temperature distribution diagram along the radial direction

The temperature is distributed parabolically along the radial direction. The surface temperature rises and the volume expands. This expansion is hindered by the low-temperature core and generates stress. The thermal stress generated can be decomposed into three parts: axial stress (σz), tangential stress (σt) and radial stress (σr). Thermal stress in three directions can be calculated based on the elastic theory. In addition to the radial stress which is tensile stress in the radius direction, the axial stress and tangential stress are in a compressive state on the surface, while the core is in a tensile state. The thermal stress and residual stress during the rolling process are superimposed, resulting in a large synthetic tensile stress in the center. When the synthetic stress exceeds the tensile strength of the material, cracks will occur in the core of the roll and even the roll may break. If there are shrinkage cavities, porosity and inclusion defects, the occurrence of roll breakage will be aggravated.

Some rolls with high hardness often burst into many pieces because the residual stress of these rolls is high. If the high residual stress peak under the surface of the roll is too close to the surface, catastrophic damage will occur due to thermal and mechanical stress, or surface cracks and damage extend to the tensile stress zone. Other factors that promote this kind of damage are: the decomposition of a large amount of residual austenite in the roll structure during the operation of the roll, which increases the internal stress along with the volume expansion. High hydrogen content in steel rollers and its harmful effects (brittle cracking); subsurface defects, especially when they are present in areas of high tensile stress.

The residual stress in the final stage of finishing rolling of a high-NiCr roll in a hot-band tandem rolling mill with a diameter of 650 mm has been measured, as shown in Figure 2. As can be seen from Figure 2, the residual stress in the axial direction of the roll core can reach 12 kg/mm2, and its ultimate tensile strength σb=19 ~27kg/mm2. Therefore, if the longitudinal stress caused by thermal stress is >7 ~ 10 kg/mm2, the roll will fracture starting from the core. This form of fracture is also called thermal shock fracture.

Figure 2 Residual stress distribution diagram of high NiCr roll

In order to avoid this kind of thermal shock fracture, when producing rolls, internal defects, such as shrinkage holes, porosity, inclusions, etc., should be minimized to improve the strength of the core material; increase the plasticity of the core material of the roll and increase the plastic deformation under stress. , reduce thermal stress; the internal structure is well graphitized, and the outer layer of high-alloy material should be thinned as much as possible to have good thermal conductivity and reduce the temperature difference between inside and outside. Reasonably formulate the heat treatment process to control the outer layer compressive stress within a certain range to reduce the core tensile stress. Figure 3 is a physical photo of a roll cracked due to excessive internal stress.

Figure 3 Actual photo of roll cracking

When the roll starts rolling, there must be a roll hot process, rolling several easy-to-roll varieties and controlling the rolling rhythm. If necessary, preheat the roll before installing the roll. Ensure good cooling conditions, strictly prohibit closed water rolling, and try to reduce the temperature gradient between the inner and outer layers; ordinary iron rollers must have a natural aging period of half a year to one year before use to reduce the residual stress in the roller. For hot-rolled thin plate rolls, it is recommended to use controllable induction heating equipment (even heating, controllable heating speed) when preheating the rolls. The preheating temperature is 380~420℃, and the preheating time is 6~7 hours in summer and 10~12 hours in winter.  The roll preheating cannot be done too hastily. There must be sufficient heat preservation time to prevent roll breakage due to excessive internal stress caused by large temperature differences. In fact, roller breakage is often caused by insufficient preheating time, forming a vicious cycle. At the same time, the replaced roll should be immediately placed in a slow cooling pit for slow cooling to avoid or weaken the generation of regenerative thermal stress.

Fatigue crack propagation and fracture

During the rolling process, the cracks on the roll are affected by bending stress. If one of the cracks becomes very deep, it cannot be eliminated during heavy rolling. When rolling continues, the cracks expand rapidly under the action of alternating thermal stress and periodic bending stress. At the same time, due to the embedding of iron oxide scale and the corrosion of cooling water during the rolling process, the expansion of cracks is accelerated. When the crack fatigue expands to a certain depth, the effective diameter of the roll becomes smaller and roll breakage occurs.

Figure 4 is a photo of the roll fracture caused by the expansion of annular cracks in the roll of a stacked thin plate. It can be seen from Figure 4 that the large carbides in the surface layer of the roll body are brittle and cause thermal cracks under the action of alternating thermal stress. During the rolling process, fatigue expands into annular cracks, and the annular cracks cause the roll body to break. When observing the fault characteristics of the broken rolls of ordinary cold hard laminated thin plate rolls, it was found that the crack distribution in the outer layer of the middle part of the roll body was mostly vertical and axial. This is due to the high temperature in the middle of the roll body, the largest bending moment, and the frequent bending fatigue effects during the rolling process. As rolling proceeds, this type of crack gradually expands along the circumferential direction and forms annular cracks, reducing the effective load-bearing cross-section of the roll body.

Figure 4 Roll fracture caused by the expansion of annular cracks in the work roll

To prevent roll breakage caused by crack expansion, the most important thing is to ensure that the roll has a certain weight and take necessary detection methods. Methods such as coloring flaw detection, magnetic particle flaw detection, eddy current flaw detection, etc. can also be used to strengthen the surface treatment of the roll to improve the fatigue strength of the roll.

Roll breakage caused by casting defects

There may be defects such as shrinkage cavities, looseness, slag inclusions, pores, and white spots inside the roll. On the one hand, these defects reduce the strength of the material, and on the other hand, they reduce the effective load-bearing cross-section. They often become the source of cracks, which gradually expand under the action of periodic stress, and finally cause the roll to break. Figure 5 is a physical photo of a roll broken due to the expansion of the defect source. As can be seen from Figure 5, there are inclusions and fatigue cracks in the cross section, which leads to a reduction in the strength of the roll. The expansion of fatigue cracks in turn causes a reduction in the effective load-bearing section. The combination of the two causes the breakage of the roll.

Figure 5 Defect source causes roll breakage

Roll breakage caused by abnormal structure

Insufficient inoculation of cast iron rolls, unreasonable distribution of ingredients, too low casting temperature, too strong thermal conductivity of cold type coating, and too thin coating thickness may cause the white layer of the roll to be too deep or the entire cross-section to be transcrystalline. It will seriously reduce the strength of the roll and cause the roll to break.

The ductile iron roll has poor spheroidization or spheroidization decline. When the graphite in the matrix exists in flakes, it seriously destroys the continuity of the metal matrix, thereby reducing the effective cross-sectional area of the metal matrix and causing local stress concentration after the roll is stressed. The local stress peak caused by the destructive effect of flake graphite can reach 10 to 20 times the principal stress. The smaller and longer the radius of curvature of the end of the flake graphite, the greater this value. In this case, the strength of the roll matrix can only be effectively utilized by 30% to 50%. Usually the strength of ductile iron is above 300 MPa, while the strength of high-strength gray cast iron does not exceed 270 MPa, and that of ordinary gray iron is below 200 MPa.

The spheroidization recession is less obvious on small and medium-sized rolls, but is quite prominent on large ductile iron rolls. When the molten iron remains for a long time, the graphite in the upper neck of the roll cannot spheroidize. Large blooming graphite or dendrite graphite will appear (as shown in Figure 6), causing a great loss of matrix strength and seriously reducing the strength of the cast iron roll. Therefore, for ductile iron rolls, the spheroidization in the molten iron must be sufficient, and the sedation time after the spheroidization treatment should be appropriately shortened, which is beneficial to reducing the decline of ductile iron.

Figure 6 Aspheric graphite structure

In high NiCr and high Ni acicular ductile iron and forged steel rolls, if the amount of martensite or retained austenite is too high, austenite will transform to martensite during the rolling process, which will increase the internal stress of the roll. It can also cause the roller to break.

A roll breakage accident occurred in a Φ880 mm high-chromium iron roll used in a steel rolling mill, and the roll body was broken into two parts. When producing this roller, the heat treatment was not carried out properly due to the furnace. The amount of sorbite was obviously insufficient and the amount of martensite (retained austenite) was too large. As a result, during the use of the roll, the structural transformation stress increases, causing the roll to break. Its metallographic structure is shown in Figure 7.

Figure 7 Metallographic diagram with excessive amount of residual austenite

For forged steel rolls, component segregation (macro-segregation and micro-segregation) is a factor that cannot be ignored. Segregation makes the entire casting structure non-uniform. During heat treatment and quenching, the difference in cooling rate along the cross-section intensifies the non-uniformity of the structure. Upper bainite is prone to cleavage and brittle fracture may occur. Ferrite has low fatigue strength and is prone to fatigue cores; when carbide or graphite is severely segregated, it will also seriously weaken the strength of the roll and become a source of fatigue. When the stress is large enough, it will cause the roll to break.

Roll breakage caused by roll neck quality

The roll neck material is unqualified. The roll neck material has low yield strength and poor fatigue resistance, causing fatigue cracks in the roll neck under the action of large rolling force or alternating bending stress. For casting rolls, due to insufficient feeding of molten steel during condensation, shrinkage holes are formed at the roll neck, which reduces the strength of the roll neck. It cannot withstand excessive torsional moment loads during steel rolling, and it is easy to break the roll neck, break the torx head and keyway. . In order to reduce such defects, measures such as adjusting the chemical composition, increasing material strength, appropriately increasing casting temperature, increasing scum capacity, and sequential solidification have been taken.

Roller breakage caused by use

Roller breakage caused by operating errors

The main reasons are: during the steel rolling production process, rolling low-temperature steel and blackhead steel or not operating according to the operating procedures, blindly increasing the reduction amount and reducing the number of rolling passes, resulting in excessive rolling pressure, resulting in roll breakage. After closed water rolling, cooling water is supplied too quickly, causing the roll to break due to excessive thermal stress. The rolls are installed incorrectly, and the rolls are stressed unevenly during operation, causing local overload and roll breakage; when cold rolling thin plates, the roll pressing force is too large, and the torque is greater than the rolling torque, and the shaft head may be twisted when starting the rolling mill.

Roller breakage caused by improper design

The designed fillet radius at the root of the roll neck is too small, causing stress concentration and improper roll material selection, resulting in roll breakage.

Roller breakage due to equipment reasons

The components in the transmission part of the rolling mill are worn, causing additional bending stress in the roll neck and shaft head, causing the roll neck or the transmission end to break. The bearing burns out of the box, causing the roll neck temperature to be too high and the roll neck to break. In high-speed rolling mills, the high-speed bite impact of the rolled piece causes torsional vibration in the transmission system, which can increase the stress load several times. The stress generated by torsional vibration can also cause fatigue fracture of the roll. This phenomenon becomes increasingly serious as rolling mills move towards high-speed, high-power, and multi-armature motor drives, and should be considered in terms of roll strength.

Conclusion

Plate and strip steel mills should pay enough attention to the selection and use of rolls. The quality of use is directly related to the operation rate of the production line and the quality of steel plates. It must start from both manufacturing and use. Improve the intrinsic quality of the rolls to prevent abnormal scrapping, while ensuring that the rolls are replaced as planned and used rationally to extend the service life of the rolls and reduce roll consumption. Roll manufacturers and users should strengthen communication and constantly summarize experience to meet the production needs of medium and thick plates.

 

MM GROUP is one of the professional roll manufacturing base in China, which supply all kinds of large-size rolls for iron and steel enterprises with production capacity of 100,000 tons of all kinds of hot strip mill rolls, section mil rolls, rod mil rolls, cold rolling m rolls, casting and forging backup rolls.

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