Study On Hot Stamping Process And Phase Transformation Mechanism Of Ultra-high Strength Steel Auto Body Structure Parts

Automotive lightweight is the national major scientific and technological needs,but also an important direction of development.One of the main methods to realize the lightweight of automobile is the hot stamping technology of ultra-high strength steel(UHSS).The hot stamping process of UHSS involves many fields such as temperature field,stress and strain field,material flow velocity field,phase change field and so on.Also the law and mechanism of deformation are complex.Therefore,it is necessary to study the forming law and deformation mechanism of UHSS hot stamping,promoting the application of UHSS hot stamping technology in lightweight body.Based on the theoretical analysis,finite element simulation and experimental methods,the constitutive models coupled with dislocation density and damage is established;the influence of hot stamping process and mechanism of phase transformation of UHSS auto body structure parts were investigated.The results of this study can promote the theory and technology development of hot stamping of ultra high strength steel,and provide reference for hot stamping production of auto body structure parts.The influences of deformation temperature and strain rate on hot deformation behavior of UHSS were investigated through the isothermal tensile tests.A viscoplastic constitutive model coupled with dislocation density was built,of which the material constants were determined by using forward Euler integral method and genetic algorithm from the experimental data.The model can preferably reflect the high temperature flow behavior of UHSS under the various deforming temperatures and strain rates.Based on the above mentioned model and introducing a damage evolution equation with the multiaxial stress-state effect,a viscoplastic forming limit model coupled with damage was developed.The material constants of the model were determined according to the isothermal bulging test data under 820~oC.The prediction effect was good at the left side of forming limit diagram(FLD)by using the developed model,while there was some discrepancy at the right side of FLD.Through the model the FLCs of UHSS under different deformation temperatures and strain rates can be predicted.To reduce the tool surface temperature and improve the production efficiency,a low-temperature hot stamping process was proposed,where in the deformation temperature is 500~oC~650~oC.The low-temperature hot stamping numerical simulations were performed on auto body components with different structures,as well as the conventional hot stamping process.The influences of the twodifferent hot stamping process on microstructure and forming properties were investigated.The results showed that the conventional hot stamping process can obtain components with better forming properties;while the low-temperature hot stamping process can obtain components with higher martensite fraction;the parts with bottom feature of convex surface and inclined wall feature were fully formed and quenched.The hot stamping experiment was performed on the typical auto body component.The influences of UHSS hot deformation amount on microstructure transformation,mechanical property and element distribution were investigated.A theoretical calculation model of the mechanical driving force during martensite transformation under multiaxial stress-state was developed.The results showed that the hot stamping deformation promoted martensite transformation in two aspects:reducing the chemical driving force;offering perfect sites for the nucleation of martensite.