| With increasing global concern on energy saving,emission reduction,and safety issues in the field of the automotive industry,major economies all are in higher demand of lightweight materials and relevant precision forming technology.Due to its advantages of low price,ultra-high-strength,stable structure,and properties,advanced high strength steel still occupies a very important market in the age of vigorous development of automotive nowadays and may maintain its important position in the next few decades.However,problems such as large dimension deviation and assembly difficulty occur during cold forming process of high strength steel plate with a higher grade because of ultra-high springback,bringing great challenges to industrial production.Although hot stamping technology could greatly reduce the springback of quenchable steel such as boron steel,it is not suitable for the forming of dual-phase steel with insufficient hardenability.It has been reported that the warm forming process can effectively reduce the forming springback of dual-phase steel with insufficient hardenability,and does not affect its elongation and strength after formed,and thus is promising in the precision forming of advanced high strength steel.Mainly speaking,the paper focused on springback in warm forming of DP980 dual-phase steel plate,the thermo-mechanical properties,cyclic deformation characteristics,and springback behavior of materials involved in warm forming are systematically,deeply analyzed,and investigated.The main research work and corresponding conclusions are drawn as follows:1.The evolution of flow stress,elastic modulus,and Bauschinger effect of DP980 steel plate with temperature variation was studied by uniaxial tensile test and uniaxial loading-unloading-reloading cyclic tensile test under different temperature conditions.The results show the flow stress of the DP980 dual-phase steel plate is less sensitive to temperature at 25?-300? and the flow stress decreases obviously when the temperature exceeds 300?.The initial elastic modulus decreases significantly with the increase of temperature,and the attenuation of chord modulus is wavy with the increase of temperature.Bauschinger effect is gradually weakened with the increase of temperature,and the higher the temperature is,the higher the proportion of Bauschinger effect in the inelastic strain is.While the greater the inelastic strain is,the higher the nonlinear elastic recovery of the material is,namely,the greater the springback is,indicating that the inelastic recovery of the material is mainly provided by Bauschinger effect at elevated temperature.2.Based on the framework of small deformation elastoplastic constitutive theory,the Mises plasticity in combination with the isotropic/nonlinear kinematic hardening model(IHKH model)was established.In this study,the coefficients of the IHKH model were calculated by the least square method and the optimization method,respectively.On the one hand,the least squares method is to directly obtain the IHKH model coefficients by fitting monotonic tensile test.On the other hand,the IHKH model parameters were calibrated with Zang's novel approach based on three-point bending test conducted on pre-strain sheets indirectly,which is based on Hooke-Jeeves algorithm in Isight platform in alliance with Python and ABAQUS software.The result shows that the IHKH model parameters obtained by Isight optimization can better predict the springback profile.In addition,by extracting the stress-strain history of the top element in the geometric center of the sample during the full-field simulation of three-point bending test with pre-strain,the cyclic tensile-compressive stress-strain of DP980 dual phase steel sheet from 25? to400? is obtained.It could be concluded that DP980 steel plate has a very strong Bauschinger effect at 25?,and the Bauschinger effect of the material gradually weakens and finally tends to saturation as the temperature increases.3.The SHS model,viz.,linear combination of swift model and Hockett-Sherby model,with different hardening rates,was adopted to extrapolate the plastic strain level of DP980 steel sheet to 50%.According to the extrapolated stress-strain curves,the material parameters of the temperature-dependent isotropic hardening model(IH model),nonlinear follow-up hardening model(KH model),and mixed isotropic/nonlinear hardening model(IHKH model)were obtained,and the thermo-mechanical coupling constitutive model based on the Mises yield theory was established.Lastly,the springback behavior of U-shaped parts at 25 ?,250 ?,350 ? and 400 ? was successfully predicted.It is found that the temperature effect can weaken the tension-compression asymmetry of the stress in the thickness direction of the thin sheet by investigating the stress distribution in the curling section of the side wall of the U-shaped part.4.As for the problem of obvious anisotropy of DP980 steel sheet caused by temperature effect,non-uniform gap drawing method is proposed to improve the non-uniform deformation of cylindrical parts in the drawing process,an analytical prediction model of earing profile considering the plate directivity and non-uniform gap was established,and the distribution function of non-uniform gap was also obtained.On the one hand,taking the room temperature deep drawing of DC04 steel sheet as an example,the influence of the uniform and non-uniform clearance on earing and springback was investigated through the method of combining deep drawing test and finite element simulation.it can be found that the non-uniform clearance deep drawing process can not only reduce the height of earing profile,but also be effective reduce the springback of opening and torsion in the process of deep drawing-cutting.On the other hand,taking the warm deep drawing of DP980 steel sheet as an example,the influence of temperature effect on earing and split-ring springback was studied,and the prediciotn ability of Hill48+IH modl and Mises+IHKH model on springback were discussed.The results show that the earing tendency of DP980 steel sheet is extremely sensitive to the non-uniform clearances,and the Mises+IHKH model can better predict the split-ring springback of DP980 steel sheet than the Hill48+IH model.In addition,the research results also pointed out that the coupling effect of temperature effect and non-uniform clearance can effectively reduce the stress gradient of the material inside and outside of the split-ring,thus reducing springback. |
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