Study On Failure Stress Type Criterion Of Warm Forming For Advanced High Strength Dual-Phase Steel Sheet

Advanced high strength Dual Phase Steel（Dual Phase steel for short）is widely used in automobile body strengthening parts for its advantages of high strength and good toughness.However,it is easy to crack and other defects near the edge of the die in the process of stamping and forming body parts on account of poor plasticity and poor formability at room temperature with the steel.The researches has shown that the sheet metal warm forming stamping technology can effectively improve the material elongation and improve its forming property.Taking DP590,DP780,DP980 and DP1180 dual phase steel as the research object to research the fracture failure of dual phase steel under the condition of warm forming.The material properties and failure behavior under the condition of warm forming were explored by establishing the forming limit stress diagram under the condition of warm forming which as its failure criterion in the process of stamping.The main work includes:（1）Stress-strain curves and related mechanical parameters of four advanced high-strength dual-phase steel plates were obtained by unidirectional tensile test at temperature range of 473～873K and strain rate of 1×10^-4s^-1.Swift constitutive model was used to describe the stress-strain curves of high strength steel sheet at different temperatures,and the simulation model was established to verify the reliability of the mechanical parameters and constitutive model.（2）The fracture strain and other data were obtained by tensile shear test of four kinds of dual phase steel plates with different angles at different temperatures.And the relevant mechanical parameters near the fracture location were obtained with the numerical simulation.The undetermined parameters of the MMC fracture criterion at different temperatures were calibrated by the method of equations,and the correlation law between the parameters and temperature was explored.The proposed criterion was applied to the failure prediction of two-phase steel sheet.The reliability of the established criterion was verified by the tensile bending stamping test of U-shaped parts.（3）The strain and stress states of the fracture region of the dual phase steel plates under shear fracture were investigated through the tensile shear test of the plane shear specimen.The analytical calculation method of the equivalent strain and stress triaxiality in the ligament region of the shear specimen under plane shear was derived with the analysis of specimen center deformation and numerical simulation.And the B-W shear fracture failure criterion based on the stress triaxiality was established.（4）The Forming limit diagram（FLD）of different sheet metal at different temperatures was obtained by the expansion experiments of the two phase sheet metal at different temperatures.The forming limit strain data obtained were then converted by the formula to obtain the forming limit stress of sheet metal.Forming limit stress diagram（FLSD）,which is a criterion for forming sheet metal under warm forming condition,was formed and used as a basis for judging whether fracture failure occurs in sheet metal forming.The stress criterion was verified by the box stamping test which simulates that the edge of body sheet is prone to crack during the stamping process.