Phase Transformation Modeling Of Q&P980 Steel And Its Application Under Complex Loading Paths

With the rapid growth of the automobile industry,the incidence of traffic accidents is increasing.Based on this,people’s requirements for the safety performance of automobiles are also constantly improving.Strength-ductility balance is an important factor to evaluate the safety performance of automobiles.Automotive structural parts need to have sufficient strength to withstand various forces,and at the same time,they need to have sufficient plasticity to ensure that no fracture occurs during the forming process.The first generation of high-strength steel for automobiles has a low strengthductility balance ratio.It has high strength but low plasticity.Therefore,the first generation of high strength steel for automobile is prone to cracking during stamping process.The second generation of high strength steel for automobile is developed by adding a variety of minor elements on the basis of the first generation,which has the shortage of high cost.Q&P steel by “quenching-carbon partitioning” process can be obtained by martensite,ferrite and appropriate amount of retained austenite composite structure.This microstructure ensures high strength and high plasticity of the material.In recent years,Q&P steel has become a typical representative of the third generation of advanced high strength steel,and has been widely used in modern automobile industry.It is worth noting that Q&P steel improves the strength and plasticity of the material through the transformation of retained austenite to martensite.However,in the actual production of body parts,the thermomechanical treatment process that the raw material undergoes can lead to extremely complex phase transformation behavior during the forming process.In this thesis,the effects of temperature,strain rates and stress state on martensitic transformation and macroscopic mechanical behavior of Q&P980 steel were systematically studied.The main research contents are as follows:(1)The effects of temperature,strain rate and strain path on the martensitic transformation of Q&P980 ultra-high strength steel were investigated.The mechanical properties of Q&P980 steel obtained by tensile tests at different temperatures(25,50,70,110,140 and 180 °C),strain rates(0.0002,0.002,0.02,0.1,1,5 and 10 /s)and stress states(pure shear,uniaxial tension,plane strain and Biaxial tension)provide basic parameters for the subsequent simulation.In this thesis,an intermediate tensile test was set up to obtain specimens with different strain.Subsequently,the evolution of retained austenite was obtained by microstructure analysis.(2)Based on the Sherif model,the martensitic transformation kinetic model was established by coupling the factors such as temperature,strain rate and strain path.Firstly,to account for the inhibitory effect of temperature rise on martensitic transformation,a temperature-dependent martensitic transformation model was established.Then,considering the promotion and inhibition of martensitic transformation at different strain rates,the martensitic transformation model coupled with temperature and strain rate is established.Finally,the stress triaxiality and Lode angle parameters are used to quantify the effect of the stress state during the forming process,and martensitic transformation model incorporating temperature,strain rate and stress state is developed.(3)The mechanical properties under different conditions were obtained by tensile tests at different temperatures,strain rates and stress states.A macro-micro coupled stressstrain model considering martensitic transformation was established.The flow stress under uniaxial tension and pure shear deformation modes under the same equivalent plastic strain was compared.The factors affecting the flow stress were analyzed,including martensitic transformation strengthening,high temperature softening and strain rate strengthening.Finally,the established macro-micro coupled stress-strain model can accurately describe the special stress-strain behavior of Q&P980 steel.(4)Users’ subroutine VUMAT in ABAQUS/Explicit was implemented to described macro-microscopic coupled stress-strain model based on phase transformation.The volume fraction of retained austenite under complex loading paths was predicted using the established finite element model.The prediction results confirm the stability and reliability of the established model.Concomitantly,this thesis explored the martensitic transformation law and the relationship between “deformation-phase transformation” of Q&P steel under complex loading conditions by using microstructure detection and other methods.