Work Roll Wear And Thermal Deformation In Hot Strip Mill

Flatness was an important quality index in the hot strip mill.Roll wear and thermal deformation is a significant factor influencing the flatness and shape of the strip.The paper was one part of systematic Research on Roll Wear Model of 1780 Hot Strip Mill in cooperation with Bao Steel Co.It was carried out in the following areas including the properties of the material,heat treatment technology,microstructure and performance,properties of thermal fatigue and failure mechanism of the roll.A series of roll wear test on site has been done in order to optimize the roll wear model.Moreover,the simulation of temperature field and the thermal deformation of rolls were studied by the finite element methods.(1)It was focused on material properties of roll for stainless steel strip in this paper.Casting microstructure and compositions of high chromium cast steel roll were also studied.Equilibrium and non-equilibrium solidification phase diagram of high chromium cast steel was analyzed by Thermo-Calc(TCW2)software.Solidification process of high chromium cast steel by centrifugal casting process was also analyzed systematically.Casting microstructure variety law during the process of heat treatment was summarized.The results from above studies provided the relevant data for selecting suitable material,controlling casting microstructure,and optimizing heat treatment process of roll.(2)One main factor researched was the influence of austenitizing temperature and holding time on microstructure,compositions and hardness of roll,analyzed using SEM,EDS,XRD and hardness measurement.The results indicated that when austenitizing heat treatment process of roll was kept at 1030℃ for 1.5 h,quenched before air-cooling,then tempered at 520℃ for 10～30 min,the hardness of high chromium cast steel was reached to 740～760 HV after tempering treatment.The sample used in the experiment showed a homogeneous microstructure and good hardening effects.(3)The microstructure and properties of high chromium cast steel during thermal fatigue process were further studied.At first,the relationship between thermal fatigue cycles and the main crack length were obtained by constrained thermal fatigue testing.And then,the influence of grain structure and composition segregation on thermal fatigue crack extended was analyzed by SEM and EDS.The results showed that the surface condition of roll could make a difference in the nucleation,path and method of thermal fatigue crack;and the thermal cyclic upper temperature and heating rate had great influences on fatigue crack growth rate(FCG rate);the material properties and the change of microstructure also had a discernible effect on thermal fatigue behaviors.(4)The surface crack configuration,interior crack configuration and fracture characteristics of the samples of high-speed steel roll were observed and analyzed using SEM,EDS and hardness meter.The type,shape,microstructure distribution,fracture and crack-growth feature of carbide of HSS roll were examined.The factors influencing fatigue crack formation,growth and hardness,wear resistance of material were evaluated.The results indicated that thermal fatigue cracks on the surface of HSS roll were caused by serious thermal strain,when HSS roll suffered hot and cold rapid alterations.It also concluded that the nucleation,path and method of fatigue cracks were related to thermal fatigue or contact fatigue stress.In order to reduce the fatigue cracks nucleation and growth,the content of inclusions in roll should be decreased,the shape of inclusions should be refined,while the shape and distribution of carbide in roll microstructure should be improved.(5)A temperature field model of roll during hot strip mall was developed based on finite element MSC.Superform analysis software.First,the temperature filed and thermal crown of roll were simulated.Then,comparative analysis with the result of experiments was performed.Finally,correlation modified coefficients of thermal crown model were optimized and adjusted.Based on the analysis above,the difference between measured and predicted results was negligible,and the prediction precision of roll thermal crown was improved.The results were the foundation for improving designed precision of strip shape,enhancing stainless steel shape quality and raising product stability,but also provided useful basis data for the study of thermal wear.(6)Experimental curve of roll wear profile was acquired by collecting data from finish rolling mills of 1780 Hot Strip Mill in BAO steel Co.It was apparent from the curve that roll force influencing index a and constant term of wear conversion coefficients bw were major factors influencing roll wear model of hot strip mill.Calculated parameters of roll wear mathematical model were optimized by iSIGHT software based on comparing with experimental data,by which calculated error of model was decreased.The ultimate optimized result was that,when a was 1.02,bw was 2.58×10-6,roll wear profile curve of calculation was very close to experimental curve,which improved the predicting precision of roll wear.(7)The changeable contours of roll gap and poor surface quality of roll by roll wear had a causation relationship with affections and surface quality of the strip.The condition can be improved by developing a reasonable rolling plan to transfer roll wear,using CVC technology and PC technology to make roll wear uniformly,and applying lubrication and fully cooling condition to reduce wear.Meanwhile,it was well known that the stability of oxide film play an important role on decreasing friction coefficients between roll and strip steel,improving wear resistance of roll and improving size precision and surface quality of strip steel.Accordingly,the spalling behavior of oxide film of HSS roll can be prevented by optimizing rolling temperature,cooling between stands and cooling between rolls to control surface temperature of strip steel efficiently.