| With increasing demand for vehicle weight reduction and passenger safety improvement,hot stamping of boron steel is increasingly becoming popular.Hot stamping technology improves the formability,decreases the spring-back and guarantees the high strength by heating the boron steel to the austenitization and then forming and quenching simultaneously in the cooled die.Traditional hot stamped components own the same tensile strength up to 1500MPa in each section,however,the new requirement that parts should be produced with different mechanical properties in various region is put forward to enhance the crashworthiness in recent years.To manufacture variable strength body parts with improved collision performances,a new hot stamping process of boron steel by partition heating was proposed.The following aspects were elaborated in order to investigate hot stamping process by partition heating.A hot stamping simulation experiment was performed using a furnace and a flat die to investigate the effect of heating temperature and heating time on the mechanical and microstructural properties of as-quenched boron steel.Based on thermal expansion experiment,the influence of heating process on the austenitization of boron steel was studied,obtaining the relationship between the soaking temperature and the austenitization degree,The dynamic austenitization model of boron steel coupling non-isothermal and isothermal heating process was established.The related material constants were identified making use of a genetic algorithm-based optimization tool.Uniaxial tensile tests were carried out on a GleebleTM 3500 thermo mechanical simulator applying different heating temperatures ranging from 700℃to 900℃.The effect of the heating temperature,deformation temperature,and strain rate on the flow stress was analysed,showing that the peak flow stress increased at increasing volume fraction of austenite.An improved unified viscoplastic constitutive model taking into account the austenitization degree was developed.Finite element models of cup-shaped and M-shaped parts hot stamping by partition heating was established.The differences of temperature,thickness,and Vickers hardness in different zones of the part during forming and quenching stages were analyzed.The effects of hot stamping parameters such as friction coefficient,stamping velocity,blank holder and heating temperature on the forming and mechanical properties of the tailored M-shaped part were investigated.Response surface models reflecting the relationship of tensile strength and elongation with heating temperature and heating time in both high and low temperature regions are established and analyzed.Multi-objective optimization is performed by using non-dominated sorting genetic algorithm Ⅱ(NSGA-Ⅱ)to obtain the optimal frontiers,and the optimal heating parameters are selected according different demands for mechanical properties in high and low temperature regions.Heating temperature of 885℃ and heating time of 4 min are selected as the optimal parameter in high-temperature region for higher tensile strength,and heating at 670℃ for 2.5 min is chosen in low-temperature region for higher elongation.The M-shaped part with tailored properties was hot stamped using partition heating according to optimal heating parameters,whose tensile strengths and elongations are 1572 MPa and 9.41%in high-temperature region and 554 MPa and 26.73%in low-temperature region,respectively.Through this study,the feasibility of hot stamping by partition heating to produce components with tailored properties was verified’ and some key technologies such as optimization of process parameters,austenitization and constitutive modelling and simulation were studied,which provides a new method for the manufacture of the tailored components. |
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