【作者】 但文蛟；

【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】 随着汽车工业的迅猛发展,汽车与环境、汽车与能源之间的矛盾日益尖锐。为解决这一问题,汽车轻量化已成为目前汽车工业进一步发展的必由之路。但在汽车轻量化过程中,确保汽车安全性是一个重要任务。因此,汽车用钢板向高强度钢板方向发展势在必行。TRIP钢是一种新型高强度钢板,它在塑性变形过程中存在TRIP效应,即TRIP钢在塑性变形过程中残余奥氏体向马氏体转变,这一过程赋予了TRIP钢良好的力学性能,使得TRIP钢板具有高强度、高塑性等特点。这同时也带来这一新型钢板“怎样才能在未来车身覆盖件广泛应用”的实际问题。目前,国内外对TRIP钢的研究主要集中在材料学领域,如TRIP钢的材料制备及合金元素对其力学性能、相变微观运动机制影响等。但是,对于TRIP钢在冲压成形过程中相变诱发塑性行为尚缺乏深入有效的研究,特别是在冲压过程中冲压速度、初始变形程度对TRIP钢相变诱发塑性行为的影响尚未见文献报道过。本文以TRIP钢板在冲压成形过程中的应用作为主要目标,开展TRIP钢板相变诱发塑性行为研究。考虑板料冲压过程中的速度效应和预应变效应,建立体现冲压特性的马氏体相变动力学模型;基于TRIP钢多相特性,建立简单有效的描述TRIP钢应力应变关系的多相混合硬化准则;考虑TRIP钢在变形过程中马氏体体积膨胀的特点,建立考虑体积变化的各向异性屈服方程;在此研究基础上,对传统的弹塑性力学进行拓展,建立起TRIP钢本构关系,用于描述TRIP钢冲压过程的内廪关系。在上述TRIP钢本构理论基础上,研究马氏体相变对TRIP钢力学特性影响,为后续研究TRIP钢板在冲压成形过程中的力学性能变化奠定基础。最后研究冲压速度和加载工步对TRIP钢冲压成形性能的影响规律及变形控制。本文研究主要内容包括:(1)基于应变速率、预应变的TRIP钢本构关系TRIP钢中残余奥氏体的相变行为对变形方式较敏感,只有在一定的变形方式或组合变形方式下才能充分发挥其相变诱发塑性的潜能,而目前在这方面虽有研究,但是主要集中于简单实验模型上,而且以经验公式为主。如何结合板料冲压成形特征,建立和丰富TRIP钢相变诱发塑性动力学模型,是工程应用的技术基础。本文基于塑性应变诱发马氏体相变的三个经典模型,结合冲压过程中固有的冲压速度和多步成形特征,建立起冲压条件下TRIP钢本构关系。首先考虑应变速率引起的温升效应,建立起冲压速度条件下TRIP钢马氏体相变动力学模型;考虑预应变,建立多步成形条件下TRIP钢马氏体相变动力学模型。鉴于TRIP钢是由多相材料(马氏体、残余奥氏体、铁素体、贝氏体)组成,在原有奥氏体不锈钢两相(马氏体、残余奥氏体)混合硬化准则基础上,建立多相混合硬化准则,用于描述TRIP钢板在变形过程中的应力应变关系曲线。由于TRIP钢在相变过程中存在体积变化,将考虑体积效应的Miller屈服准则引入本构方程,并将其扩展到板料各向异性范围。通过实验验证和相关文献的实验结果比较可得,本文提出的考虑冲压特性TRIP钢本构关系能够很好的描述TRIP钢板冲压成形过程,这为TRIP钢板冲压成形进一步研究提供理论基础。(2)TRIP板钢硬化特性分析及分散失稳准则作为描述材料硬化特性参数之一的硬化率是用于表征材料应变强化程度。应变硬化指数直接反映了材料发生颈缩前依靠硬化使材料均匀变形的能力大小,应变硬化指数的大小不仅反映材料的塑性加工性能,而且对于冲压成形件的合理选材也具有指导意义。板料塑性成形中主要的质量问题之一是破裂,而破裂本质是拉伸失稳现象。在成形过程中,控制材料拉伸失稳是生产合格零件的前提。本文在已建立的TRIP钢本构关系基础上,先后分析了常规条件下、以及不同应变速率和不同预应变条件下,TRIP钢硬化率、应变硬化指数、稳定性等力学特性及其变化规律。在TRIP钢塑性变形过程中,TRIP钢的硬化率、应变硬化指数随着残余奥氏体体积份数和马氏体体积份数变化而变化;而马氏体的体积份数还受马氏体相变速度的影响,即应变速率和预应变等的影响。因此TRIP钢硬化率、应变硬化指数与传统方式定义的硬化率、应变硬化指数有本质不同。另外,TRIP钢在发生塑性变形时存在体积变化,从而引起TRIP钢板在发生分散失稳时要考虑体积变化对失稳点影响;在应变速率和预应变条件下,失稳点进一步产生变化。(3)TRIP钢冲压成形性能分析及变形控制为工程实践中合理有效使用TRIP钢,本文在上述本构关系基础上,结合TRIP钢成形过程中的力学特性,研究了不同冲压速度加载形式、多道次冲压初始冲压深度对杯形件最小厚度、U形件反弹角影响。结合TRIP钢板硬化率,分析了冲压速度加载形式、多步冲压初始冲压深度等对TRIP钢板成形特性的影响。基于TRIP钢冲压件不同部位(如,凸模圆角、侧壁、凹模圆角部分)在成形中的性能变化规律,并初步建立TRIP钢冲压成形性能的控制策略。更多还原

【Abstract】 Along with the prosperity of automobile industry, the conflicts between the automobiles and the environment and the energy concern the entire world. In this circumstance, lightening-weight products become vital goal in the future development of automobile. At the same time, it is also very important to keep the secure property of automobiles in the lightening-weight process. Therefore, further development of high strength sheet steels in automobile has been a trend. As a kind of new steel, TRIP steel is characterized by high strength and high ductility because of the TRIP effect brought by transformation process during which retained austenite transforms into martensite with better mechanical properties. However, another practical problem is how to apply the new steels to the automobile body widely. At present, studies on the TRIP steel at home and abroad are mainly conducted in the material science domain. The extant research focuses on TRIP steels material preparation, the influence of alloy element on its mechanics and its motivation performance of microscopic movement, and so on. Whereas, few studies are conducted on the transformation-induced plasticity behaviors in the sheet forming process. Especially, extant literatures had not yet aimed at the study on influence of punching speed and forming step on the transformation-induced plasticity behaviors in a punching process.This thesis aims at studying the transformation-induced plasticity behaviors and the following application of TRIP steel to the sheet forming process. Considering forming speed and pre-strain effect, a novel martensite transformation models with punching behaviors are established in a sheet forming process. Based on TRIP steel’s multiphase behaviors, a simple and valid law of multiphase mixed hardening is given for describing stress-strain relationship of TRIP steels. Considering the martensite’s volume change, an anisotropic yield equation is constructed. Based on the above studies, the further work is to establish a new constitutive equation of TRIP steels to describe the inner relationship of a punching process by the expansion of classical theory of elastic-plastic mechanics. With the support of the constitutive equation of TRIP steels, effects of martensite transformation on the material behaviors of TRIP steels are investigated. And the research effectively supports the following study on the material behaviors of TRIP steels in a punching and forming process. Finally it is arranged for exploring the influence of punching speed and loading step on forming behaviors of TRIP steels and establishing a controlling of deformation in forming process.The main content of this thesis is divided into three parts:(1)TRIP steel constitutive equation considering strain rate & pre-strain The retained austenite transformation in TRIP steels is sensitive to deformation state. So the potential of TRIP effect is opened in the specific or combined deformation states. Although some authors studied the behaviors of TRIP steels, they worked mainly on the simple experimental models which derived chiefly from experiential expressions. So it is necessary to establish a transformation-induced plasticity novel model in a sheet forming process for the engineering application.On the basis of three classical models of strain-induced transformation, here establishes novel constitutive equations of TRIP steels sheet forming process with considering the forming speed and muti-step forming. Specifically, considering temperature increasing resulting from strain rate, martensite transformation models for TRIP steels are established under the condition of different forming speed. On the other hand, considering pre-strain, martensite transformation models for TRIP steels are established under the condition of multi-step forming. At the same time, based on the existing mixed hardening laws of austenite steels with only two phases (martensite, retained austenite), a new multi-transformation mixed hardening laws are built here to describe stress-strain relationship of TRIP steels with four phases (martensite, retained austenite, ferrite and bainite) under the condition of strain rate and pre-strain respectively. Considering the volume change of TRIP steels during the transformation process, new constitutive equation is characterized by the embeddedness of the Miller yield law with volume effect and the new anisotropic yield law. By comparison our experimental results to the extant literatures’results, it is found that the new constitutive equation of TRIP steels proposed here describe the TRIP sheet forming process much better than the existing models. This result provides a rigid theoretical basis for the further studies on TRIP steel sheet forming.(2)Study on hardening behaviors and diffuse instability of TRIP steelsAs one of the parameters with describing the hardening behavior of materials, hardening rate, the slope of stress-strain curve, represents the strain strength level. Strain hardening exponent denotes the capability of uniformity deformation before necking of material. The value of strain hardening exponent not only reflects the plastic working range but also signifies correct selection of the material. Fracture is one of the key quality issues in the sheet forming process. In essence, the fracture is the result of plastic instability. Controlling plastic instability of the material is prerequisite to manufacture qualified products. Based on the constitutive equations in this thesis, the material properties of the hardening rate of TRIP steels, strain hardening exponent and plastic instability are explored under the usual condition, strain rate condition and pre-strain condition respectively.In the TRIP steels’plastic deformation process, the hardening rate and strain hardening exponent change with the different share of the retained austenite volume and the martensite volume which affected by the martensite transformation, namely, by strain rate and pre-strain. Therefore, the hardening rate and strain hardening exponent of TRIP steels are distinguished from those defined in traditional manner. Moreover, because of the volume change deriving from the martensite transformation in plastic deformation of TRIP steels, it is necessary to consider the impact of the volume change on the instability point while happening diffuse instability of TRIP steels. The instability point changes farther under the strain rate condition and the pre-strain condition respectively.(3)The influence of different forming condition on the formability of TRIP steels and deformation controlling in forming processIn order to make the effective use of TRIP steels in the engineering practice, this part aims at studying the effects of forming velocity loading paths and initial stamping depth in multi-step forming process on the minimum thickness of cupulate-part and springback angle of U-part on the basis of the constitutive relations established in the thesis while considering the mechanics properties in the TRIP steels’sheet forming process. Also, this part explores the effects of forming velocity loading paths and initial stamping depth in multi-step forming process on the sheet formability by hardening rate. Different position (such as punch radius, side, and die radius) in the forming sheet is found to have different distribution to the formability of TRIP steels. New methods are given here to control the formability better in TRIP steels sheet forming process.更多还原