Research And Realization Of Key Algorithm For Hot Forming Simulation Of High-strength Steel Sheet Based On KMAS

In order to achieve the purpose of lightweight vehicles and meet the car’s crash safety requirement,the hot stamping process of high strength steel sheet has been recognized by industry.The forming simulation technology can provide strong supports for optimizing processing and solving difficulties during process.Finite element(FE)methods for numerical simulation of sheet forming are mainly based on increment and deformation theories.The deformation method can quickly and roughly evaluate the formability of workpiece just according the shape and size of hot stamping products,as well as little process information.And it is mainly used in the initial stage of product and die design.The increment method has higher simulation accuracy,but needs longer time for calculation,and more die and process information.The method requires users to have better professional competence and it is mainly used in the middle or late stages of product and die design.Both methods have high practical value.Compared with the cold stamping process,the hot stamping simulation process is more complex because not only the geometry,materials,and nonlinear contact problems are involved,but also the temperature and other factors.This paper focuses on the research of simulation algorithms for ’the high temperature forming stage’ of hot stamping process.Currently,the FE algorithms applied to the hot stamping process all belong to the incremental method,but the hot forming FE algorithms based on deformation theory applied in the initial stages of product and die design is lacking.In view of the above problems,the inverse forming algorithm based on deformation theory for hot forming of high strength steel sheet was proposed in the paper,and the inverse forming algorithm was also realized on the basis of one-step inverse forming module for cold forming of KMAS.The main contents of this paper are as follows:(1)For analyzing the high temperature thermodynamic properties of high strength boron steel sheet 22MnB5,the thermodynamic tension test was designed,and a plurality of test pieces were stretched by ’Multi-Functional Thermal Tensile Testing Machine’.And the flow stress curves of boron steel at different temperature and strain rate were obtained,which were the preparations for building high-temperature constitutive equation of 22MnB5 and having the forming simulation.(2)One-step inverse forming simulation algorithm based on deformation theory for hot forming of high strength steel sheet was proposed,the research includes the deformation process of object,the method of expressing the strain of an object,and based on the above,the geometric relationship of the method was established.And with the experimental results of the thermodynamic tension test,the improved Norton-Hoff constitutive equation of 22MnB5 in the state of high temperature austenite was built.Taking the triangular element as the research object,the node internal force vector was deduced according to Kirchhoff hypothesis.In the stamping process,for the external force on the sheet metal,the force of punch,die,binder and drawbead were expressed by equivalent node external force,and the loading history was ignored.With the method of Newton-Raphson(N-R),the iterative format of the method was presented.And the process of One-step inverse forming algorithm for hot forming of high strength steel sheet was summarized.The initial field prediction algorithm of parts with complex curling and the automatic determination algorithm of stamping direction parts were proposed.On the basis of the KMAS/One-step cold forming module,one-step inverse forming algorithm for hot forming of high strength steel sheet was implemented.(3)The hot stamping experiment was implemented for the typical hot stamping product,and the hot stamping forming process and process parameters were designed reasonably,and got the test pieces of acceptable quality,which had reasonable hardness,uniform thickness,there was no any cracks and wrinkles.The one-step inverse forming algorithm of KMAS for hot forming were used to simulate the typical hot stamping products,at the same time,the commercial software DYNAFORM based on the incremental theory was also used for the corresponding simulations.Comparing the two simulation results,the experimental results showed that the simulation results of the proposed algorithm in this paper basically reached the expected.The one-step inverse forming algorithm is suitable for the initial stage of hot forming product and die design,and can roughly assess the formability of hot stamping product.