| Adopting (ultra) high strength steels to manufacture car body parts not only can reduce car weight and fuel consumption, but also can make the car more safety. It is the most effective way to realize the lightweight car body and enhance the safety of the collision, simultaneously. But (ultra) high strength steels stamping at room temperature are more vulnerable to fracture and severe springback. Especially, when the strength of the steels achieve to 1500 MPa, the conventional cold stamping technology almost impossible to forming. Therefore, how to realize the high precision stamping of high strength steels has become an urgent technical problem needs to solve. Hot stamping of ultra-high strength steels is considered to be the effective way to solve the above problem. Its process principle is:heating the boron steel to austenitizing (900～950℃) temperature, make its fully austenitizing, then move quickly to a cold mold which equipped with a cooling system to stamping the part. The blank quenched in the die and get fully martensite structure. The strength of the part after quenching can achieve to 1500MPa. In this paper, the engineering and science problems of hot stamping process of boron steels were be mainly discussed.The thermal expansion tests were conducted on the Gleeble-3500 thermo-physical simulation system. The influence of heating speed, heating temperature and soaking time on the high temperature formability of boron steel was studied. The results show that:under the circumstance of guarantee the production rhythm, the best forming performance specimen is the rapid heating one; The influence of heating temperature on the high temperature formability of boron steel is obvious; soaking time has significant influence on the peak stress of boron steel and has little influence on the ultimate strain. A unified kinetics model of the non-isothermal austenitization considered the influence of heating rate, based on the theory of austenite nucleation and growth,was established. Material constants in the kinetics model were determined and optimized through a genetic algorithm. The model can describe the austenite transformation kinetics curves of boron steel and accurately predict the fraction of the austenite under different heating rates.The response surface models of tensile strength, yield strength and elongation of hot stamping parts were established. The considered parameters are austenization temperature (800-1000℃), austenitizing soaking time (60-540s), initial deformation temperature (560-800℃) and tool temperature (20-220℃). After obtaining the response surface models, the influence of process parameters on the mechanical properties of hot stamping parts were investigated. And the process parameters optimized by a multi-objective genetic algorithms NSGA-II. The optimization process parameters can obtained hot stamped parts with optimal mechanical properties. The optimization results can provide experimental basis for hot stamping process parameter selection and theoretical guidance.A set of device was designed and used to measure cooling curve of boron steel and temperature rise curve of die. The heat transfer coefficient between the die and boron steel has been calculated. The influence of pressure and thickness of oxide on the interfacial heat transfer coefficient were studied. The study can provide theoretical basis to calculate the temperature of blank and die in hot stamping and die quenching procedure, and provides data basis for accurate calculating the phase transformation in hot stamping.Thermal expansion experiments of the boron steel HC1500HS were carried out on the Gleeble-3500 thermo-physical simulation system. The influence of cooling rate and deformation on the phase transformation of boron steel HC1500HS were investigated. The critical cooling rates of the steel were determined and the dynamic austenite continuous cooling transformation curve (DCCT) was drew. Based on the alloy thermodynamics theory, the kinetics of non-isothermal ferrite and bainite formation were described which can be used to predict the phase transformation in the continuous cooling process of boron steel HC1500HS.A fully coupled thermo-mechanical-metallurgical FE model was established for a door anti-collusion bumper of boron steel HC1500HS in hot stamping process. The microstructure of boron steel was calculated. The characteristics of temperature, microstructure and hardness distribution on the door anti-collusion bumper were investigated. The experiment verified the efficiency of the thermo-mechanical-metallurgical FE model. The microstructure evolution in hot stamping process can be predicted and the mechanical properties of hot stamping parts can be controlled. The effects of deformation temperature, die temperature, holding force and holding time on the microstructure and mechanical properties of door anti-collusion bumper hot stamped part were investigated by the coupled FE model. |
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