| With the concept of "energy conservation and emissions reduction" people have a new request for future development of the car: lightweight, high security and high impact resistance. Advanced high strength steel and the emergence of the corresponding hot stamping technology can be realized in a certain extent people to the requirement of the vehicle. As a kind of advanced high strength steel, boron steel hot forming theory and technology got extensive attention of the researchers.In order to improve the forming performance of UHSS sheets, the hot stamping for the quenchable boron steel sheet was presented and developed in recent years. At the beginning of the process, the blank is put into a furnace, heated up to an austenitization temperature, austenitized for about five minutes to get a homogeneous austenitic microstructure, then transferred quickly to a cooled die for forming and quenching simultaneously, and finally receives a full martensitic structure with an increase in hardness and strength. During the process, the material undergoes three physical changes in microstructure, ferrite+pearlite in blank, austenite in high temperature, and martensite after quenching. By comparison with cold forming proess, the hot stamping process has advantage in the forming of AHSS and UHSS, such as less deformation resistance, good ductility for the blank in high temperature, good formability in complex shapes,good dimensions, little spring-back and high strength.This topic is for BR1500 HS high strength steel plate based on theoretical and experimental research of other scholars. Adopt Gleeble-3500 hot simulation experiment machine and finite element simulation technology to study the mechanism of high strength steel plate deep transformation. This paper mainly focuses on the BR1500 HS TTT curve of boron steel, phase transformation kinetics model, the constitutive relation and finite element simulation analysis of several aspects of heat treatment.TTT curve can intuitively reflect the supercooled austenite amount of each phase transition in the process of isothermal transformation relationship with time. TTT curve is judgment organization transformation in the process of heat treatment and the mainly theoretical basis of calculating of the amount of organization transformation. More important is that it can provide theoretical guidance for the establishment of hot stamping process parameters. In this paper, through Gleeble3500 tester using the method of thermal expansion, and got a BR1500 HS boron steel under different temperature of phase transformation start and end points. By using optical microscope observation of sample status determine material tissue types under appropriate temperature. According to the results of test and calculation, draw the BR1500 HS TTT curve.Considering the nucleation rate is related to time and crystallization kinetics of universal equation are given, this paper chooses Avrami equation to calculate the diffusivity phase transformation kinetics model. A Avrami type equation = -e[ 0.0248939*1(4 --TV)xp 20] was acquired to characterize the evolution of phase transformation volume fraction in a given temperature. For without diffusion model transformation, time counter rotating variable doesn’t work. The temperature of the sample determines the austenite phase variable. Chooseing Marburger experimental research method to calculate the non proliferation phase change of expression, a equation = -e[ 0.0248939*1(4 --TV)xp 20] was acquired to characterize the evolution of phase transformation volume fraction in the martensite temperature range.Through the study of hot tensile experiment of BR1500 HS boron steel sample, and basised on the of the stress-strain curve, using multiple linear regression and polynomial fitting build the constitutive equation to accurate description BR1500 HS alloy thermoplastic rheological behavior. In high temperature and low temperature, this paper established a Arrhenius constitutive equation and Johnson-Cook constitutive equation respectively.Results show that the calculated data are in relatively well with the experimental data. Austenite+ferrite, pearlite, bainite and martensite constitutive equation of boron steel can be very well in the macroscopic mechanical behavior in the process of hot stamping. The rheological models to predict phase region and phase transformation kinetics model import the finite element analysis soft. Results have predicted the strength of steel during hot deformation and pressure maintaining workpiece internal microstructure change. Experiment results and the obtained forecast stress-strain curve input to the software, realized the simulation of microstructure. It provided a more reliable theoretical basis for High-strength steel plate hot stamping process. |
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