Reasearch On Constitutive Parameter Identification And Forming Limit Establishment Method Of Tailor Rolled Blank

An effective way to realize energy saving and emission reduction is automobile lightweight,the related new materials,new structures and new processes have become the key technologies to promote the development of the automotive i ndustry.Tailor Rolled Blank(TRB),which has excellent mechanical properties and lightweight ability,is an advanced high strength steel produced by flexible rolling process.TRB provides a new idea for the design of thin vehical structures.In order to analyze TRB's stamping formability,a series of research is carried out around the method to establish the constitutive model and to predict the forming limit using theory.(1)Study on the design method of TRB tensile specimen:Based on the various thicknesses and properties of TRB,a new design method of tensile specimen with various width is roposed.The new specimen can balance the difference of TRB's thickness and property.Then,the stress-strain curve of all thicknesses from TRB's transition zone is obtained directly by tensile test using digital image correlation technology(DIC).The results show that the new specimen can guarantee the tensile strain of all thicknesses not less than 0.125.(2)Study on the establishment method of general hardening model:According to the stress-strain data,the ability of four empirical equations namely Ludwik,Hollomon,Swift and Voce to describe HSLA340's strain hardening behavior is compared.Result shows that Swift equation is the best.Then,TRB's general hardening model was established by introducing a rolling thinning coefficient?.Finally,the bending test of TRB hat-shaped structure and its numerical simulation were used to verify the general model.(3)Study on inverse method of anisotropic parameters:The micro indentation test was carried out along rolling direction,the vertical rolling direction and thickness direction respectively.The load-displacement curve and the maximum heap height after unloading were obtained.The results showed that the load curves and the maximum heap height in different directions were obviously different.Then,the numerical simulation of indentation test is established and the Hill48 yield criterion was applied to simulate the anisotropy behavior.Based on the experimental data and simulation,the inverse research on anisotropic parameters was carried out.Finally,the reliability of the inverse method is verified by testing the Lankford coefficient in three different directions(0~o,45~o,90~o).(4)Study on the establishment method of TRB's forming limit:The equations for calculating forming limit was established by combining different yield criterion with Instability theory.The experimental data of TRIP780 indicate that the M-K groove model combined with the BBC2005 yield criterion could predict forming limit of high strength steel sheet best.The transition zone of TRB was dispersed as a combination of some uniform thickness plates and the M-K model was used to calculate all plates's forming limit.The result showed that with the increase of rolling thinning rate,the forming limit curve of TRB decreases.Finally,the principal strain is expressed as the two polynomial function of rolling thinning coefficient and secondary strain,and the three-dimensional forming limit surface of TRB was established.