Inconsistency Of Microstructure And Mechanical Properties Of Dual Phase Steel And Its Effects On Sheet Forming

Due to the excellent comprehensive performance,Dual Phase(DP)steel has been the most widely used Advanced High Strength Steel(AHSS)for manufacturing of automobile structural parts in recent years.However,the microstructure and mechanical properties of DP steels from different production lines may be remarkably inconsistent as a result of different chemistries and manufacturing processes applied.The inconsistency of microstructure and mechanical properties brings significant impacts on the application especially sheet forming of DP steel.In this study,the inconsistency of microstructure and mechanical properties of dual phase steel and its effects on sheet forming are investigated on DP780 steels from different production lines through field investigation,theoretical study,experimental study and finite element method.The main researches and achievements are as follows:(1)It is found that the mechanical properties,chemical compositions and microstructures of DP780 steels from different production lines are significantly distinct by uniaxial tensile test,composition testing and metallographic observation.The differences in the stress strain relationship and elongation of DP780 steels with different chemical compositions and microstructures are analyzed by the micromechanics-based finite element model established.Based on the micro-mechanism of failure,the indicator of the failure element ratio is proposed to evaluate the elongation of DP780 steel with martensite critical failure stress criterion.The effects of carbon content,martensite volume fraction and ferrite grain average diameter on the mechanical properties of DP780 steel are obtained quantitatively.(2)Simulated annealing experiments are designed and carried out in the laboratory based on the actual industrial production process.The effects of continuous annealing process parameters on microstructure and mechanical properties of DP780 steel are obtained.The results show that the effect of intercritical temperature is continuous while the sensitive interval and non-sensitive interval should be considered in terms of the effects of slow cooling temperature,over-aging temperature and strip speed.The influence degree is defined to quantitatively compare the effects of the influencing factors,which finds that the over-aging temperature has the most influential effect on the tensile strength while the effect of the slow cooling temperature ranks second and the intercritical temperature has a minimal impact.The influence degrees on elongation of all the three factors are similar.(3)It is found that the drawability and stretchability of DP780 steels from different production lines are significantly distinct by two representative stamping processes,the round-cup drawing test and the dome test.Finite element models for the two tests are built to analyze the stress and strain evolutions during the forming processes.Based on the results of experiment and the finite element analysis,the deep drawing and bulging forming limit of DP780 steel under different strength coefficient K,strain hardening exponent n,Lankford coefficient r and thickness t are predicted and obtained quantitatively with the Cockcroft-Latham ductile fracture criterion.