Kinetics Model Of Deformation Induced Martensitic Transformation And Springback Behavior For QP Steel Sheet Forming

With the development of automobile industry,auto-body lightweight technology becomes the most important issue to auto-body manufacturing.Advanced high strength steels（AHSS）has been widely used in automobile industry and becomes key lightweight materials in auto-body.The 1^st and 2^nd generation AHSS are unable to achieve good balance between low cost production and high outcome of strength and elongation.Therefore,the Quenching and Partitioning（QP）process has been proposed to overcome the drawback.Retained austenite inside the low cost QP steel would transform to martensite with the assist of deformation,which brings about both high strength and preferable ductility.Therefore,QP steel has become the typical representative of 3rd generation AHSS,which attracted special attention from automobile industry.The retained austenite to martensite transformation of QP steel brings about good balance between strength and ductility,but also brings about extra problemes.Due to the complex behavior of deformation induced martensitic transformation（DIMT）,the related flow behavior might be more complex than other AHSS,and its springback prediction method might also be different.Thus,it is still important to study the relation between martensitic transformation,mechanical response and springback,which would be helpful to understand the mechanical behavior and popularize the application of commercial QP steels.In the present study,the DIMT,DIMT related mechanical behavior and springback of QP980 steel sheets have been investigated.Firstly,in order to consider characteristics of temperature-strain rate coupling and non-linear strain path under sheet metal stamping process,a transformation kinetics model considering the interaction between temperature,strain rate and stress state was developed.Secondly,the DIMT dependent plastic constitutive equation was developed to describe the complex flow behavior of QP steel.This model would be helpful to reveal the relation between martensitic transformation and stress-strain relation.Finally,the impact of martensitic transformation and DIMT related flow behavior on springback behavior of QP steel was investigated.A new springback prediction method has been proposed for QP steel.Main contents and innovations of this dissertation can be explained as follows:（1）Experimental study on the deformation induced martensitic transformationCombined with the universal tensile tests machine and Split Hopkinson tension bar apparatus,the DIMT behavior of QP steel under quasi-static,moderate and high strain rate has been investigated.The infrared thermography system has been used to monitor the deformation induced heating.Based on the newly proposed heat balance equation,the mechanism of deformation induced heating on DIMT has been revealed.The effect of strain rate was found to suppress and promote the DIMT under quasi-static strain rate and moderate strain rate,respectively.Combined with DIC and sheet metal cyclic loading system,the effect of stress state on DIMT are able to be investigated.The experimental results show that the transformation rate strongly depends on the stress state with the largest for equi-biaxial tension,progressively decreasing for plain strain tension,uniaxial tension,compression and simple shear.（2）A transformation kinetics model considering temperature,strain rate and stress stateThe Sherif transformation kinetics model has been modified to considering the combined effect of temperature,strain rate and stress state,which well reproduced the DIMT behavior of QP steel under temperature-strain rate coupled condition and non-linear strain path condition.The new model revealed that the non-linear strain path itself are not found to affect the DIMT.It is the evolution of stress triaxiality and lode angle parameter that affect the DIMT behavior during non-linear strain path.The comparison between experimental and model predicted results demonstrates the model’s good accuracy,which lay a solid foundation for the later study on DIMT dependent flow behavior and springback prediction.（3）A martensitic transformation dependent plastic constitutive equationExperimental flow behaviors of QP steel under different deformation modes,temperatures and strain rates show that the more complex flow behavior of QP steel than other AHSS.Based on the martensitic transformation behavior,the mechanism of deformation mode dependent flow behavior and temperature dependent strain rate sensitivity of QP steel have been explained.A newly proposed plastic constitutive equation has been developed to consider the contribution of DIMT,thermal softening effect and strain rate strengthen effect on flow stress.In comparison with existing plastic constitutive model,the new model better reproduced flow behavior of QP steels.（4）Springback behavior and case studyThe sheet metal cyclic test system and constitutive model calibration method have been developed.For materials models calibrated from uniaxial tension–compression test（TCT）and cyclic shear test（CST）,their limitation for the springback prediction of QP steel has been discussed,which mainly caused by the DIMT behavior.To overcome this drawback,a new materials’parameter calibration method has been proposed for QP steel.Cases study of springback prediction for U-bending part and top side beam of auto-body verified the validation of present new method for QP steel.Overall,this dissertation systematically studied the DIMT and DIMT dependent flow behavior of QP steel.Based on this study,the springback prediction method for QP steel has been developed.The achievements of this dissertation provide the guidance for springback prediction and engineering application of QP steels.