| The internal microstructure,content and distribution of metal materials determine the macro properties of materials.The initial temperature,strain rate and deformation in the forming process are closely related to the microstructure evolution processes such as dynamic recrystallization and dynamic recovery in the deformation process and the phase transformation in the cooling process,which has an important impact on the microstructure morphology after forming.The strengthening and toughening mechanism of the third generation automotive steel is a research hotspot in the current automotive industry and steel industry.This paper takes the third generation automotive steel docol 1500 bor as the research object,analyzes the relationship between deformation process and microstructure evolution based on Gleeble thermal simulation compression experiment and EBSD microstructure characterization,and establishes a thermal mechanical microstructure coupling model with finite element analysis software to analyze the hot stamping process of A-pillar reinforcing plate,Combined with the macro and micro mechanical properties test,the relationship between microstructure content group and fracture toughness of experimental steel is studied.The main work of this paper is as follows:（1）The thermal simulation compression experiment of docol 1500 bor steel was carried out in the temperature range of 900～1100℃and the strain rate of 0.01～20s-1.The rheological stress curve was drawn according to the obtained stress-strain data,and the metallographic samples were prepared.The variation laws of rheological stress and microstructure evolution under different conditions were analyzed.The parameters such as deformation activation energy and temperature compensated deformation rate factor were solved by Arrhenius model,and the constitutive equation was established,After verification,the error is in the range of 0.5～8.4%,which establishes the basis for the material model of subsequent simulation.（2）Based on the stress-strain data obtained from the thermal compression experiment,the hot working diagram of docol 1500 bor steel is drawn by using the material dynamic model and Prasad instability criterion.It is obtained that the processing performance of the material is the best when the temperature is 980～1020℃and the strain rate is 0.01～0.5s-1.The recrystallization volume fraction and Schmidt factor distribution under different processes obtained from the EBSD experiment are analyzed,By reconstructing the high temperature parent phase and analyzing the distribution of its typical slip surface,the accuracy of the hot working diagram is proved.（3）The hot stamping process of A-pillar reinforcing plate is studied through finite element analysis software combined with hot working diagram.The effects of hot stamping process on stamping stress,thinning rate and martensite volume fraction are analyzed.It is found that the best stamping process is initial plate temperature 950℃,stamping speed 20mm/s and holding time 15s.Combined with nano indentation experiment and fracture toughness simulation,the fracture toughness of the experimental steel in rolling direction,normal direction and transverse direction is studied.It is concluded that the experimental steel is relatively easy to crack in rolling direction,and the fracture toughness decreases with the increase of martensite volume fraction,but the overall change of fracture toughness is small.The accuracy of the model is proved by quantitative analysis of Schmidt factor through EBSD experiment. |
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