Determination Of Friction Coefficient Of Dual Phase Steel DP590 And Its Research On Forming Limit And Application

The vehicles will change towards lightweight in the future,and the most important method of realization is to adopt advanced high strength steel.However,galling and thinning cracking often occur during forming of auto body parts.The dual phase steel DP590 was as the research project in this paper,and the comparisons of forming limit between theoretical prediction and experiments were explored which wuould be as the instability criterion for simulation of deep drawing.Under the conditions of importing rheological curves and friction coefficient by the best surface morphology,the thinning rates and fractures of deep drawing parts were analyzed based on the comparisons between the simulations and experiments.In this paper,the main research contents of DP590 have now been summarized as follows:(1)Friction test was carried out with different loads and sliding speeds according to the contact stress between sheet metal and die.The results showed that,normal load and sliding velocity have influence on average friction coefficient of DP590.With the increase of load,the average friction coefficient of DP590 firstly increases then decreases,and with the increase of sliding velocity,the average friction coefficient of DP590 decreases.Surface morphology of DP590 sheet metal was observed by SEM which helped to confirm the best value of friction coefficient under best surface morphology.(2)Strain hardening index n and hardening coefficient K were calculated by uniaxial tensile test combined with classic Hollomon hardening curve equation and the least square method.Considere Decision Rule was derived by critical state between strengthening and necking during the process of tensile,which can find out the instability point of the material and paved the way for the theory to predict the FLD of DP590.Anisotropy indexes of DP590 were calculated which were as the imported parameters for the theoretical prediction of the forming limit.(3)Forming limit theoretical prediction model of dual phase steel DP590 was deduced according to the Hill-Swift instability theory combining the Hill48 yield criterion with Hollomon hardening curve equation.Rigid die bulging experiment was carried out to have access to the limit strain points of DP590.Compared with the one predicted by Hill-Swift instability theory,the whole limited strain points measured in experiments were higher.(4)The mechanical performance parameters such as friction coefficients,the rheological curves and forming limit diagram were imported into the finite element software Dynaform,Firstly,the comparisons of experiments and simulations were performed to verify the reliability of the numerical simulations,and then the impact of process parameters on the thinning rates of the different parts in deep drawing was analyzed.The results showed that,the blank holder forces had an influence on the thinning rates,and the flange and die fillet on the upper of deep drawing parts thickened,die fillet on the lower and cylinder wall of DP590 had been thinning on different degrees,dangerous section reached the largest thinning rates.Stamping velocity had little effects on the thinning rates of DP590 in low speeds.