| TRIP steel, in which occurs the transformation induced plasticity phenomena, has not only high strength but also good plasticity. TRIP steel has been given more attention by the international steel and auto industry in order to applying it more widely in the light auto's body project. As a kind of new type steel, the TRIP steel contains retained austenite in the room temperature because of special chemical components design and rolling technique. The retained austenite is unstable to transform to martensite during deformation. The new martensite phase has higher work hardening ability than the retained austenite, which can increase the material's plasticity. The TRIP effect gives TRIP steel excellent mechanical property to improve the deep drawing ability. However, springback for TRIP steel is more complicated due to volume inflation and phase transformation hardening caused by the transformation from retained austenite to martensite. It's difficult to predict accurately the springback of TRIP steel adopting traditional FEM. It can extend the use field and proportion and shorten the time of new product design if the prediction and controlling of the springback of the TRIP steel are resolved satisfactorily.Based on the comparison and evaluation of TRIP steel research in the world, the transformation, work hardening character, deep drawing ability and springback are investigated systemically. The martensitic transformation kinetics basing on the strain road and the multiphase TRIP steel's flow stress model are developed at first. And then they are introduced in the FEM model to predict the evolution of transformed retained austenite, the mechanical properties and the deep drawing ability for TRIP steel. At last the evolution of Young's modulus with transformation during deformation is investigated, and the prediction of springback considering for the varying Young's has higher precision than the traditional FEM regarding the Young's modulus as constant. To fulfill the above research objectives, the following five aspects of efforts are performed:(1) Modeling the martensitic transformation kinetics basing on strain roadThe monotonic loading test, which represents stress-stain states in sheet steel forming, includes single shear, uniaxial tension, plane strain and equal stretching tests. The relation between the volume fraction of retained austenite and plastic strain in deferent loading mode can be obtained by the experimental results. The macro stress states are expressed by the stress triaxiality. Then the model of martensitic transformation kinetics basing on macro stress state is developed. The stress triaxiality is deduced to the strain road by the relative plastic theory, when the model of martensitic transformation kinetics basing on strain road is brought forward to apply in the sheet steel forming. The martensitic transformation kinetics is groundwork to model the mechanical property of TRIP steel.(2) Flowing stress modeling and mechanical characteristics of TRIP steelThe mechanical characteristics of TRIP steel are very deferent from the traditional high strength steel's because of TRIP effect. Combined the transformation kinetics, a micromechanical model is developed by the mean field theory. The stress-strain relation, special work hardening, necking and forming limited curve of the TRIP steel are investigated by the mechanical model. And effect of the volume fraction and stability of the retained austenite on the mechanical behavior of the TRIP steel is analyzed. An accurate mechanical model is a base for the FEM.(3) FEM analyses and process optimization for TRIP steel sheet formingTo make clear the reason that the TRIP steel has a good drawability, the computer prediction for the evolution of transformation is carried out. The transformation is more rapid at the corner than at the else positions. The mechanical driving force of transformation comes from accumulated strain energy. Upon strain-induced transformation, this energy is absorbed and dislocation pile-up is relaxed. Therefore the crack probability decreases. On another hand, the local thincking is smaller for TRIP steel than traditional high strength steel. Basing on M-K theory, FLD is established taking account for the TRIP effect to evaluate deep forming ability for TRIP steel. Comparison of the utmost depth between that considering TRIP effect and without TRIP effect is utilized to indicate the deep forming improving action. The effect of process factors on transformation of TRIP steel is analyzed and the main factor influent on transformation is determined. An approximate modeling method is developed to optimize forming process, involving design of experiment, adaptive response surface method and solution strategy. Then two examples, optimization of forming process for a cup and optimization of forming process for a rectangle, are given aiming to safety FLC and certain amount retained austenite. Through comparison with the experimental results, not only the wrinkling can be avoided, but also the retained austenite has enough to ensure subsequent deformation after optimization. The optimization procedure contributes to the application of TRIP steel sheet forming. (4) Prediction and robust controlling for the springback of TRIP steelThe predict precision was very low for springback of TRIP steel calculated by linear elastic removing loading. To solve the problem, the Young's modulus evolution with new transformed martensitic volume fraction has been studied by experiments. Taking into account the variation of Young's modulus for TRIP steels in springback simulation, the prediction is accord better with experiments than that using traditional FEM. Under the condition of ensure predict precision, varying blank holding force is optimized by the way combining FEM and optimization method to control the springback. To reducing the fluctuating springback, engineering robust design has been used in application. The optimization and robust design basing on computer simulation provides new means for enhancing the part shape reliable.
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