Research On Flow Behaviors And Microstructure Evolutions During Warm/Hot Deformation In 20CrMnTi Low-alloy Steel

Forging parts,of which China is the largest producer,are widely used in the automobile,energy,ship and aviation industries.It is in urgent need of technical upgrading,facing pressures of cost,environmental protection and market.Due to the coupling effect of temperature field and stress field,the manufacture of warm forgings and hot forgings is more complex.In order to improve the production efficiency and quality,it is very important to investigate the macroscopic mechanical behavior and microscopic metallurgical phenomena during plastic deformation,and it is also a hot topic for the research of domestic and foreign scholars.The research object of this paper is 20CrMnTi low-alloy steel,which is one of the most representative warm forging and hot forging materials,and is widely used for gear,shaft and flange parts.Aim at the warm/hot forming process,the flow behavior and microstructural evolution of the material were systematically studied through the combination of experimental research and theoretical analysis.It would provide a theoretical reference for controlling the microstructure and properties of the material after deformation.In this paper,the flow bevavior of 20CrMnTi low-alloy steel druing warm deformation（873-1173 K）was first investigated by iso-thermal compression tests.The compression strain-stress curves showed that the flow stress at 1023 K was lower than that at 1073 K under four studied strain rates,with an abnormal stress-temperature region emerging.It was found that this abnormal phenomenon appeared in the austenite-ferrite dual-phase temperature region,while ferrite occurred at low temperatures（873,923 and 973 K）and austenite occurred at high temperatures（1123 and 1173 K）,respectively.The constitutive relationships of single ferrite and single austenite were constructed by a dislocation model coupling Arrhenius equations,which was verified by microstructural obserbations.Based on the single phase’s constitutive relationship,the abnormal stress-strain curves were manifested through a modified iso-strain mixture law,in which a higher strain rate sensitivity of ferrite at intercritical temperatures was taken into consideration.In the end,a unified constitutive model which could adequately describe the flow behavior of 20CrMnTi low-alloy steel during warm deformation.Meanwhile,the flow behavior during hot deformation was investigated though iso-thermal compression tests in the temperature of 1173-1323 K.By analyzing the stress-strain curves,four characteristic stresses（yield stress,eak stress,saturated stress and steady-state stress）as well as two characteristic strains（crictical strain and strain with maximum softening rate）were obtained and modeled with initial grain size and Z-parameter.The dislocation model was established to describe the constitutive relationship exhibiting work hardening（WH）and dynamic recovery（DRV）.A new kinetics model of dynamic recrystallization（DRX）was introduced and modified to quantitify the stress reduction from the WH-DRV model.Compared with the conventional Averami-type kinetics model,the model could exactly exhibit the velocity of DRX process.Eventually,the proposed constitutive model could accurately predict the flow stress during hot deformation.The microstructure of austenite at high temperatures,which cannot be reserved at room temperatures,determine the properties of its product phase in 20CrMnTi low-alloy steels.Consequently,an accurate method as well as its automatic praogram was developed to reconstruct local orientations of parent austenite（γ）phase from EBSD maps of the martensite（^′）microstructure.Firstly,according to classic K-S orientation relationship,the solution spaces for the orientation relationship are refined,which simultaneously improves the calculation accuracy and efficiency of reconstruction procedure.Secondly,the local orientations around parentγgrain boundaries were more accurately determined using a newly proposed reconstruction criterion.Compared the existing research,the reconstruction procedure in this paper was able to accurately obtain orientation gradients inside grains,grain boundaries as well as twin boundaries.This would be benifitial to the analysis of microstructural evolution.The effects of deformation temperature and strain rate on microstructural evolution of austenite in 20CrMnTi were analyzed by the reconstruction program.In order to analyze the microstructural evolution of austenite during hot deforation more comprehensively,an austentite-stablized steel with a stack fault energy（SFE）similar to20CrMnTi was selected.The isothermal compression tests（1323 K）were employed to understand the roles of strain rates(0.001,0.01,0.1,1,and 10 s^-1)and twinning in flow behaviors and microstructure evolution.Using EBSD and TEM analysis,it was determined that the alloy underwent discontinuous dynamic recrystallization（DDRX）and continuous dynamic recrystallization（CDRX）in low and high strain rate regimes,respectively.In the low strain rate regime,twinning contributed to the separation of the bulged region for dynamic recrystallization（DRX）nucleation,andΣ3 twin boundaries mainly formed by‘Growth accident’during the subsequent growth of nuclei.In the high strain rate regime,twinning did not require boundaries migration,and altered the orientation of boundary fronts,thus promoting the extension of the DRX area.Grain size exponents in low and high strain rate regimes were calculated to be-0.68 and-2.9,respectively.It was also confirmed that different DRX mechanisms result in distinct evolution of grain size,and the dividing point was determined to be the strain rate of 0.1 s^-1.The established constitutive model,DRX model and grain size evolution model of20CrMnTi were wrote into Deform-3D platform.Then,the forging process of a spur gear was realized in the FEM software.Besides,processing experiments were conducted to verify the simulation above.The process of filling tooth cavity in the simulation was consistent with that in the experiment.Obtained by models above in this paper,the prediction error of forming load was within 11%,and the prediction error of grain size was within10%.Hower,the prediction errors of conventional models（based on the Avrami DRX model）were within20%and 25%.It is verified models in this paper were more accurate to predict flow behaviors and microstructures.