【作者】 单体坤；

【导师】 张卫刚； 李淑慧；

【作者基本信息】 上海交通大学 ， 车辆工程， 2008， 博士

【摘要】 利用变形过程中相变诱发塑性(Transformation Induced Plasticity,简称TRIP)现象研制的TRIP钢兼具强度高、塑性好的特点,在汽车轻量化过程中显示出了广阔的应用前景。作为一种新型钢板,特殊的化学成分设计和轧制工艺使TRIP钢在室温下含有亚稳态奥氏体组织,在塑性变形过程中会发生向马氏体的转变,产生局部硬化,从而带来材料塑性的提高。相变诱发塑性效应赋予TRIP钢良好力学性能,改善了TRIP钢的成形性能,然而TRIP钢具有良好成形性能的同时回弹也很严重,且由于相变的影响导致用传统的线性弹性卸载预测TRIP钢的回弹精度不高,因此研究TRIP钢在塑性变形过程中的相变行为,弄清TRIP效应改善成形的机理,提高TRIP钢回弹的预测精度,对推广TRIP钢板的工程应用、缩短新产品开发周期有着重要的理论意义和实际价值。本文在借鉴和吸收国内外先进成果的基础上针对TRIP钢的相变行为、硬化特性、成形性能和回弹特性等进行了较为系统的研究。首先是建立基于应变路径的TRIP钢相变动力学模型,使其更适应于在板料成形中的应用;在此基础上利用平均场理论建立TRIP钢流动力学模型,分析残余奥氏体体积含量和稳定性对硬化特性的影响;然后实现冲压过程中残余奥氏体转变量分布的计算机预报,揭示TRIP钢具有良好成形性能的机理,并针对TRIP钢的成形工艺进行了优化;最后考虑TRIP钢变形中由于相变导致弹性模量的变化,建立弹性模量演化模型以实现TRIP钢的非线性弹性卸载,提高了TRIP钢回弹的预测精度,并通过稳健设计制定合适工艺,降低TRIP钢回弹对外部噪声因素的敏感性。本文的主要研究内容如下:(1)基于应力状态和应变路径的马氏体动力学模型遵循从简单到复杂的科学研究方法,选择典型的单调加载模式实验,获得残余奥氏体体积含量与应变量的定量关系,通过应力三轴水平参数来表征应力状态,建立了基于应力状态的马氏体相变动力学模型;并推导了应力三轴水平和应变路径的关系,建立了基于应变路径的马氏体动力学模型。马氏体动力学模型是后续力学模型的基础。(2) TRIP钢的流动力学模型和力学特性研究结合第二章建立的TRIP钢相变动力学模型,在分析TRIP钢各微观相的弹塑性变形行为基础上,利用Hill自洽理论和平均场理论推导了TRIP钢的流动力学模型,并依据所建立的力学模型研究TRIP效应影响下的TRIP钢的特殊硬化行为,分析TRIP钢中残余奥氏体初始体积含量和稳定性对TRIP钢硬化特性的影响规律。该模型是进行TRIP钢成形性能仿真研究的基础。(3) TRIP钢成形性能和工艺优化研究通过实现相变在TRIP钢冲压零件中分布的计算机预报,指出在零件危险部位由于相变量多而降低材料层错密度,降低了材料发生破裂的概率,而且由于相变强化作用,零件危险部位产生局部硬化而减小TRIP钢的局部减薄率,揭示了TRIP钢具有良好成形性能的机理;在可描述相变诱发塑性特点的成形极限预测准则基础上,计算TRIP型多相钢的最大拉深高度,与无TRIP效应时的最大拉深高度进行对比研究TRIP效应影响成形性能的程度;接着分析冲压工艺条件对TRIP钢相变的影响,找出决定TRIP效应大小的工艺因素,为工艺优化提供铺垫;最后针对杯形件拉深进行随时间变压边力优化和毛坯尺寸优化,针对盒形件拉深的随位置变化的压边力优化和毛坯形状优化,实验结果表明优化解不仅可以避免零件成形缺陷还可以保留足够的残余奥氏体来提高零件的后续变形能力,可以指导TRIP钢的成形工艺。(4)基于非线性弹性卸载的TRIP钢回弹预测和稳健控制为了弥补传统的有限元法(认为弹性模量不变)对TRIP钢回弹预测精度低的不足,首先研究了TRIP钢弹性模量随新生相体积份数变化的规律,建立TRIP钢弹性模量演化模型;把弹性模量变化历史引入仿真过程可以极大提高TRIP钢回弹的预测精度;在精度满足的条件下,通过模拟和优化设计方法的组合进行了变压边力优化控制回弹大小,通过稳健设计方法控制TRIP钢回弹波动,建立基于非线性弹性卸载的TRIP钢回弹控制的新方法,以提高零件的尺寸精度,降低生产成本。更多还原

【Abstract】 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.更多还原