Research On The Contact Fatigue Mechanism Of Large Forged Backup Roll And Optimization Of Grinding Value

Large forged backup roll is one of main parts used in rolling mills. The performance and usage status of large forged backup rolls are directly determin the quality of products, stable of equipment and the cost of production, so more and more producers gives attention on the use of it.In the rolling process of four high or six high mill,backup roll is paired with work rolls or intermediate rolls to pass rolling force and improve the bending rigidity of work roll,which can further control the quality of plate rolled.The cyclic contact stress the backup roll endured can cause the surface or subsurface contact fatigue,lead to the cracks’initiate and proporgate,result in deterioration and intensify wear of roll surface,which further can cause the spalling and breake failure of roll.In the production process of rolling, peredical preventive grinding was used to remove the cracks initiated on the surface or in the subsurface to prevent the rolling accident and economic losses result from the spalling of backup rolls.But there have little research works been implemented for the development of grinding procedure,the goal that the production efficiency improve, consumption reduce and life prolong of backup roll cannot been achive due to the change cycle and grinding amount which determined according the roll uses experience.So suitable grinding amount determining based on the studies about the distribution and mechanism of backup rolls’contact fatigue damage have great significance.Finite element model of roll system of a four high mill was established by large nolinear finite element software based on the finite element theory of contact problem.The rationality of finite element model was verificated by compared the result computed by the model with that by theory. Studies on the influence of roll diameter pairing, plate width,rolling force, bending force and shift distance on the contact stress distribution of backup roll were implemented basing on stands Fl-4 of a hot rolling mill.The central part of backup roll is straight and the edge is chamfer.The contour of work roll is consine curve.The results revealed that the contact stress on the backup roll surface is a typical’saddle’shape. There are more or less influence of roll diameter pair conditions,plate width,rolling force and bending force on the magnitude and uniformty contact stress of backup roll except work roll shift. All the factors have no influence on the distribution location of maximum contact stress on backuop roll surface.The shakedown status of backup roll under limit loaded condition was analyzed basing on the elastic-plastic mechanics theory and contact mechanics theory. The results showed that the backup roll material is in elastic shakedown status with contact stress cycles finite times.The orthogonal shear stress was selected as key stress for the assessment of subsurface rolling contact fatigue damage by comparing analysis between the octahedral shear stress and maximum shear stress with it.The subsurface rolling contact fatigue damage calculation model was established by combining the contact fatigue life of material and linear fatigue damage accumulation theory.Fatigue damage distribution of material after one time uses and numbers times uses and grinding of backup roll were analyzed.The rationality of fatigue damage distribution calculation model was verified with the rolling contact fatigue tests on the backup roll material and experiment on the dissection part from backup rolls used in production.The hardenss and half breadth value of X-ray was measured with micron indentation instrument and X-ray diffraction to clarify the property changes of backup roll material.The optical microscopy (OM), scanning electron microscopy(SEM) and transmission electron microscopy (TEM) were employed to study the contact fatigue mechanism of backup roll material.Comparable studies on the hardeness distribution along the radial direction and microstructure at different depth from surface of dissection parts of backup roll after numbers uses and grinding were implemented.Microstructure changes of backup roll material at different depth with induced by different extent of contact fatigue damage were investigated.The increase of microhardeness, decrease of X-ray diffraction breadth value and appearance of typical white etching region in the section influenced by fatigue damage etched with nital were observed.The increase of microhardeness and decrease of X-ray diffraction breadth value are result from the broken of lath martensite to nano subgrain microstructure,precipitation of nano carbide and increase of dislocation density induced by fatigue damage. While the reveals of white etching region is owing to large quantities of interfaces introduced by the broken of lager scale lath martensite to nano subgrain structure.While the fatigue damage below a certain range,the fragementation of lath martensite and increase degree of dislocation density are different with the extent of damage induced by rolling contact fatigue.The fragementation of lath martensite and increase of dislocation density are increased with the increasing of fatigue damage.Basing on the subsurface rolling contact fatigue damage calculation model and research on the mechanism of subsurface rolling contact fatigue damage,load spectrum model for the fatigue damage calculation of backup roll in a changeable cycle was established by combining the interval division of rolling specification and corresponding rolling load for specific interval.The rationality of load spectrum model established was verified with load records of rolling production.The optimal grinding value and grinding times that the maximum fatigue damage increasing to the damage tolerance limit of backup rolls used in different stands were calculated by assumpted the change tone is equal to the rated change tone.The relationship between the change tone and grinding value was analyzed.The results revealed that the optimal grinding values and grinding times that the maximum fatigue damage increasing to the damage tolerance limit of backup rolls used in different stands are different. When a backup roll was used in a definite stand in the whole life of it,there is a linear relationshio between the change tone and grinding value. The grinding value calculation model was established for the backup roll used in a definite stand.The work of this dissertation can be taken as a guidance of the suitable use, fatigue failure prevention and grinding value deterimination of backup roll used in production.There is also reference value for contact problems in other engineered application.