Modeling And Simulation Of 60Si2CrA Steel Grinding Balls During Helical Rolling And Isothermal Quenching Process

Steel grinding balls are indispensable and important product for mining,building materials,coal and other industrial sectors.Compared with the casting and forging process of steel grinding balls,the helical rolling process has many merits such as low cost,high production efficiency and low energy consumption etc.Generally,steel grinding balls are quenched after rolling and then tempered at low temperature to obtain tempered martensite with high hardness,which may make grinding balls fragile in use.To improve the match of strength and toughness and wear resistance of grinding balls,it is proposed to produce wear-resistant steel balls with martensite/bainite duplex structure by isothermal quenching after helical rolling.This thesis focuses on engineering science problems and technical difficulties corresponding to the microstructure evolution of 60Si2CrA steel during helical rolling and isothermal quenching.The dilatometric curves of 60Si2CrA steel were measured using a dilatometer DIL805A.A unified kinetics model using the internal state variable(ISV)method was derived to describe the non-isothermal austenitization kinetics of 60Si2CrA,and the above-mentioned model models the incubation and transition periods.Combined with metallography,the continuous cooling transformation diagram of 60Si2CrA was obtained.The CCT diagram of 60Si2CrA steel was determined.There are ferrite,pearlite and martensite areas in the CCT diagram,but no bainite area,indicating that the bainite can not be obtained by continuous cooling.A phase transformation kinetics model is established to estimate the fraction of ferrite and pearlite during continuous cooling process.Based on the dilation data,a shear bainite kinetics model was established using the ISV approach.The model is coupling with bainite phase nucleation and growth,and the incubation time is characterized by the proposed normalized bainite nucleus radius and normalized radius growth rate.Material constants within the three kinetics models were calibrated with genetic algorithm-based optimization methodology.Furthermore,fairly close agreement between predicted and tested data was achieved.The hot compression test of 60Si2CrA steel was carried out on Gleeble-1500D thermal simulator.The microstructure model of 60Si2CrA steel based on dislocation density was established using ISV method.Factors such as equivalent hardening,dislocation density and dynamic recrystallization were considered in the model.The critical strain was the only occurrence condition of dynamic recrystallization,which ensured the physical meaning of the model.The material constants in the model were determined by genetic algorithm.The results show that the microstructure model can effectively reproduce the flow stress and microstructure evolution of 60Si2CrA.Two subprogram interface functions,ueloop and plotv,are used to embed the microstructure model into the software of SIMULACT FORMING,and the finite element model of ?80mm steel-ball helical rolling is established according to the basic principles of pass design.The experiment of ?80mm steel-ball helical rolling was carried out.By comparing the experimental results with the finite element simulation results,it was verified that the finite element model can effectively predict the evolution of temperature field,ball size and grain size in the process of helical rolling.The results show that the effect of grain size on the phase transformation can be ignored.Through secondary development technology,the forming results of SIMUFACT were transferred to DEFORM.The transformation model of 60Si2CrA steel was developed by FORTRAN compiler in the user subroutine MSH of DEFORM-3D.The air-cooled heat transfer coefficient of 60Si2CrA was calculated by the method of reverse heat transfer.By comparing the on-line isothermal quenching experiment with the phase transformation fraction predicted by the phase transformation finite element model after rolling,the validity of the model is verified.The variation of bainite and martensite percentages with air cooling time,water cooling time and isothermal temperature was analyzed.The response surface model of the bainite volume fraction standard deviation(S.D.)and average value(Avf)of the ball with heat treatment process parameters was established.The heat treatment process parameters were optimized with an average value of nearly 15%and a higher value of S.D..The optimization results are verified by the finite element simulation.The simulation results are in good agreement with the response surface prediction values.After optimization,Avf and S.D.increase,and the distribution of bainite is more reasonable.The ball with reasonable distribution of martensite/bainite composite structure has high surface hardness,wear resistance and core toughness.With the decrease of the radius of the steel ball in use,the stress concentration is buffered by the austenite and bainite in the new surface layer,and the TRIP effect makes the hardness of the new surface layer increase,making the steel ball keep a good match of strength and toughness.The high hardness of the surface layer ensures wear resistance,and the toughness of the core reduces the ball breaking rate.