Prediction Of FLD For High Strength Steel Under Hot Stamping Based On Damage Mechanics

The forming limit diagram(FLD)is usually used to evaluate the formability of materials in CAE simulation.The development of high strength steel hot forming technology determines the necessity of studying high temperature forming limit diagram.The method of obtaining the forming limit diagram as defined by the international standard is for cold forming,there is no standard for hot forming,If the cold forming method who is the punch type is still used,the lubrication in experiment is limited.Besides,formability of materials at elevated temperatures is correlated with temperature and strain rate,a forming limit curve(FLC)can be not evaluated.Many forming limit curves are needed which can be used to analyze the formability.However,a lot of experiments mean that the cost is very high.In this paper,a new test method for obtaining high temperature forming limit diagram is presented—Biaxial tensile test method.A device which can convert the unidirectional tension to the biaxial drawing is designed.And an insulation schemes suitable for high temperature operation environment and joint rotation is proposed.Damage factor is introduced as a parameter.Custom material model is established.And LS-DYNA finite element simulation of tensile processing is used,where tensile process can be simulated.The shape and size of specimen for different location forming limit curves are designed.By means of the design of key size specimen,the flow state of strain direction of primary and secondary materials when stretched is changed.Strain paths can be changed.Finally,a set of specimens is designed.Points of forming limit under different strain paths can be got.By means of biaxial tension test,under 700? and 800?,5 test points are obtained,such as the point of the biaxial tensile strain state?plane strain state?transient strain state.At the same time,the correctness of the new method is verified.22 Mn B5 damage constitutive model based on continuum damage mechanics is established.Multi objectives genetic algorithm are used to fit the material constants in the constitutive equation.FLD predicting model is established.And comparing the forming limit curves obtained by damage constitutive model and experimental method,the feasibility and accuracy of the constitutive model are verified.The forming limit curves of 22 Mn B5 under 600?,650?,700?,750?,800? and 850? is predicted on the damage constitutive model.And at 800 ?,when the strain rate are 0.01/s,0.1/s and 1/s,the forming limit curves are predicted respectively.With increasing of temperature or reducing strain rate,forming limit diagram(FLD)will be higher.And the formability of 22 Mn B5 is increased.