Simulation Of Asymmetric Rolling And Its Application In TWIP Steel

Asymmetric rolling(ASR)has the advantages of low rolling pressure and large rolling reduction capacity.In recent years,with further understanding of ASR process,it is found that there is additional severe shear strain in ASR process.Therefore,ASR is regarded as a severe plastic deformation to produce ultrafine grained materials and attracts numerous attentions of many researchers.Twinning induced plasticity(TWIP)steels have high strength and excellent ductility which is considered to have huge potential in automobile industry.However,previous studies of the effect of ASR processing on deformation behavior are not systematic and comprehensive,thus this will affect the process design for producing ultrafine grained materials.Moreover,the studies of producing ultrafine grained TWIP steels through ASR are less studied.In this study,we modify the conventional ASR process design which is based on experience into a design which is based on simulation,then test the optimized design,finally produce ultrafine grained TWIP steel.A finite element model for asymmetric rolling commercial pure aluminum established via MSC.Marc software is used to study the deformation behavior and to investigate key issues of cross shear zone,rolling pressure and critical speed ratio during ASR process.Then we optimize the ASR process based on finite element method for ASR processed TWIP steel and finally produce the ultrafine grained TWIP steel.The correlated conclusions of this research are as follows:The finite element model for asymmetric rolling commercial pure aluminum is demonstrated to have good reliability when compared with theoretical rolling pressure and rheology of metal.And the finite element method which comprehensively considers the effect of positive and negative shear deformation is revealed to be more accurate than embedded-pin method.Although the equivalent strain of ASR is similar when compared with that of symmetric rolling,however,in the case of total shear strain,the ASR and symmetric rolling show a notable difference.As a result,it is more rational to use total shear strain as a reference for grain refinement.The area proportion of cross shear zone has the maximum value as it is restricted by the size of deformation area in rolling.As the rolling pressure has the minimum critical value under conditions of high speed ratio,the rolling pressure will not decrease with further increasing speed ratio.The critical speed ratio is related to reduction ratio,initial thickness of material and roller diameter.The critical speed ratio will increase with decreasing thickness of initial material and increasing reduction ratio.The speed ratio(SR)affects the shear strain in two aspects.With the increasing speed ratio,the area ratio of cross shear zone will increase and hence increases the shear strain of rolled piece.Nevertheless,the rolling pressure decreases significantly as the speed ratio increases.The decreased rolling pressure will lower the friction force between roller and rolled piece which results in the decrease of shear strain.Thus,there may exist an optimal speed ratio which can optimize the shear strain in ASR.The value of an optimized speed ratio is a little bit less than critical speed ratio,and thus there do exist space for optimization of speed ratio.The nano-grained(NG)high-Mn austenitic steel with average grain size of 60 nm and the ultrafine-grained(UFG)steels with grain size below 500 nm are successfully produced by combination of the asymmetric rolling and symmetric rolling method and the subsequent annealing treatment.The annealed NG steels exhibit relatively higher strain hardening and good balance of strength and ductility,which is attributed to the partial recrystallization of the nanostructures and the relatively lower stacking fault energy of the high-Mn TWIP steel.The effects of annealing temperature on microstructure and mechanical properties of TWIP steel(Fe-0.5C-18.6Mn-1.5Al-0.5Si),produced by severe symmetric and asymmetric rolling at room temperature,is investigated in this study.The results show that the grain size is significantly refined after severe asymmetric and symmetric rolling,and the ultimate tensile strength(UTS)is increased from 593 MPa to 2021 MPa.The severely rolled TWIP steel shows no recrystallization when annealed below 500?,partial recrystallization when annealed between 500-600?,and full recrystallization when annealed above 700?.With the annealing temperature increasing,the strength decreases and elongation increases.Specifically,excellent mechanical properties and UFG(Ultrafine-grained)TWIP steel with an average grain size of 500 nm are obtained when annealed at 700? with the UTS,elongation and product of strength and elongation of 1114 MPa,59.4% and 66.2 GPa %,respectively.Moreover,when annealed between 500-600?,a large number of dispersive fine-grained DO3 structured intermetallic compounds,which significantly improve the strength,are obtained.A bulk nanostructured twinning-induced plasticity(TWIP)steel with high ductility and high strength was fabricated by cryogenic asymmetric rolling(cryo-ASR)and subsequent recovery treatment.It was found that the cryo-ASRed TWIP steels exhibit simultaneous improvements in ductility,strength and work hardening.Typical microstructures of the cryo-ASR TWIP steel are characterized by shear bands and intensive mechanical nano-sized twins induced by cryogenic deformation.These mechanical nano-scale twins remain thermally stable during the subsequent recovery treatment.It is believed that the ductility enhancement and high work-hardening ability for the cryo-ASR TWIP steels should mainly attribute to the high-density pre-existing nano-scale twins.