Stress Development During Hot Staming Process And Springback Analysis

Since hot stamping process is known as its less springback than traditional cold stamping process,the phenomena of springback in hot stamping used to be paid less attention by researchers.However,it is necessary to research the springback of hot stamping,which is a nonnegligible phenomenon especially in some parts with special shapes and high requirements of precision.In addition,springback is the macro performance of releasing the inner stress of the hot stamping part.It is meaningful to research the stress development of hot stamping process to have an intensive understanding of the forming mechanism.The research is based on the springback of hot stamping.First of all,the laws of springback of different hot stamping processes were concluded.Then a thermal-mechanical-phase transformation coupled finite element model was set up to research the origin of the springback and the stress development of the whole hot stamping process.In addition,fracture morphologies of the hot stamped steel under tensile testing and trimming testing were observed.Not only the common hot stamping process but also the Q&P hot stamping process which is known as its high ductility was researched.The main contents of this research are shown as follows:1.Three factors including shape changing,temperature and phase transformation of the hot stamping parts have significant influences on the springback.Three groups of hot stamping experiments were designed with three variable processing parameters,which is the forming velocity of common hot stamping,the partitioning time of Q&P hot stamping,as well as the forming temperature of hot stamping integrated with fractional cooling.The influences of three parameters on springback were investigated,and the relationships were concluded and analyzed.2.The FEM models of hot stamping parts were established on the basis of experiments.The reliability of the model was demonstrated by the comparison between experimental and simulated results of shape changes.The model considering TRIP effects and varied Young's modules relating with temperature field was more specific under the condition of either during the low or high velocity.The effective stress of integration points through thickness and surface stress of corner elements were investigated to explain the mechanism of springback.3.A completed stress analysis of the hot stamping part was conducted with the demonstrated FEM model,and the stress distributions and developments under different forming velocities were obtained.The influence of TRIP effects and varied Young's modules on the stress development were also researched.TRIP effects can release a great deal of inner stress during the process of martensitic phase transformation.A FEM model ignoring influences of temperature filed on Young's module may have a lower calculation of simulated stress and strains.4.Fracture morphologies of quenched B1500 HS under tensile and trimming testing were observed as well as four other cold stamped steels with different strengths.The influences of stress states on fracture behaviors and morphologies of five steels were investigated.Furthermore,the fitted empirical models were derived to describe the quantitative correlations between the fracture features of morphologies and the steel strength.