Investigation Into Hot Stamping Of High Strength Steel With Tailored Tempering Properties

With the increasing requirements of the automotive industry for reduced weight and safety improvement,high strength steel hot stamping parts have been increasingly applied in the automotive industry.The hot stamping technology is widely used because of the good formability,lower forming force,less spring back and high strength parts obtained.Therefore,with the consistently increasing requirements for safety performance improvement,the fully martensitic hot stamped parts cannot meet the new requires,which consist high strength and energy absorbing.The compatibility of mechanical properties and safety performance of vehicle parts such as b-pillar has been proposed that different mechanical properties are needed in different regions of the same parts,namely tailored tempering properties(TTP).In this paper,the experiments to get tailored tempering properties,heat transferring & phase transformation,constitutive & fracture model,tribological behavior and crashworthiness have been investigated.According to the microstructure evolution,selecting the cooling rate or phase transformation path to obtain tailored tempering properties through divided dies with different temperature,different initial temperature of the blank and local tempering.The influence of the die temperature and holding pressure has been investigated.The strength of the part decreased but the elongation increased as the die temperature increased.When the die temperature is lower than 400 oC,the hardness distribution is like V-shape and the lowest hardness occurred in the transition region.The relation between the tempering hardness and tempering parameters(temperature and time)has been built.By using the prediction model proposed,it would be easier to determine the tempering temperature and time to get assigned hardness and mechanical properties.The parameters can be optimized by the prediction model instead of the trial and error method,which will save cost and time.The final microstructure,phase volume and mechanical properties are directly determined by the cooling path during hot stamping.The full-process heat transfer model was built considering the latent heat of phase change.The modified model was proposed based on the modified Li model,which was implanted into the finite element software to predict the phase volume and mechanical properties.By using the proposed heat transfer and phase change model,the parameter sensitivities were investigated to guide the real production.Using the digital image correlation technology to measure the strain over the whole specimen and the elastoplasticity constitutive models of martensite,bainite and ferrite have been obtained.By combining the experiments and numerical simulation,the parameters of fracture model have been optimized,which are very hardly to be determined using only experiments.The constitutive and fracture model of multiphase has been obtained using the liner-mixture rule and RVE(Representative Volume Elements)method.The fracture distance error between experiments and simulation results is less than 5%.The tribological behavior during the hot stamping affect the cost,product quality and producing efficiency directly,which cannot be neglected.The self-developed friction testing machine is used to measure the friction factor and wear between die and blank.Considering the process to get tailored tempering properties,the machine has been improved to investigate the tribological behavior in the condition of different initial blank temperature,different die temperature and different cooling rate.The sudden change of friction factor has been explained and the friction factor during cooling has been expressed.The purpose of get tailored tempering properties is to improve the crashworthiness but it's hard to prove it using the tests because of the high cost and long testing cycle.However the numerical simulation can be used to assist designing and verifying.In this paper,the side crash simulation of the whole car has been proceed.The intrusion distance and velocity of the assigned points have been recorded.The results show that the high strength hot stamping parts with tailored tempering properties can obviously improve the crashworthiness.The influence of transition location,soft zone material properties,transition length and transition division number on intrusion distance and velocity has been investigated.Finally,the simulation results were verified by the single B-pillar side impact tests.