IMR OpenIR
低碳钢形变-相变耦合过程的介观模拟计算
Alternative TitleMesoscopic simulation of the coupling between deformation and austenite-ferrite transformation in low carbon steels
肖纳敏
Subtype博士
Thesis Advisor李依依
2008-05-23
Degree Grantor中国科学院金属研究所
Place of Conferral金属研究所
Degree Discipline材料加工工程
Keyword热形变 奥氏体/铁素体 晶体塑性有限元 蒙特卡罗 动态相变
Abstract为了满足经济建设和社会发展的需求,新一代钢铁材料的研究得到普遍重视。这其中一个重要的发展方向是在不提高其成本的前提下尽量提高其强韧性。显微组织的细化和均匀化是同时提高材料强度和韧性的主要手段。对于铁素体-珠光体钢来说,利用塑性形变和奥氏体-铁素体相变之间的相互作用是细化晶粒的一种有效方式。因此,研究形变-相变耦合过程的热力学和动力学,对于设计材料成分和制备加工工艺以获得所需要的材料性能具有十分重要的意义。本文采用介观尺度计算方法来研究钢铁材料中的形变-相变耦合过程。 本文首先在介观尺度建立了多晶奥氏体高温热形变的晶体塑性有限元模型。模拟结果表明,奥氏体在经历塑性形变时,其应力、应变和储存能的空间分布很不均匀,同时近邻晶粒的取向关系在很大程度上影响了奥氏体的内部微观形变行为。进一步分析表明,这种不均匀性与各个滑移系的运动密切相关。塑性形变导致的另外一个特性是奥氏体的晶界面积随着形变量的增大而不断增加。晶体塑性有限元模拟得到的晶界面积增长率明显比均匀形变几何模型的计算结果要高。 本文结合晶体塑性有限元和蒙特卡罗方法,建立了顺序耦合介观计算模型,并模拟了奥氏体在非再结晶区的塑性形变对奥氏体-铁素体相变的影响。结果表明,奥氏体的塑性形变能明显加快随后的奥氏体-铁素体相变动力学。塑性形变对碳原子在奥氏体内的长程扩散以及铁原子在相界面处的短程扩散都有促进作用。此外,奥氏体在未再结晶温度区间的塑性形变能提高铁素体的形核密度,有效细化铁素体晶粒。影响铁素体形核密度的主要因素有三个:塑性形变增加了原始奥氏体的晶界面积;奥氏体晶粒内部的部分高储存能位置为铁素体提供了额外的晶内形核位置;奥氏体晶界上的铁素体形核率明显增加。另外,由于铁素体的核心主要集中于奥氏体晶界和晶内的高储存能位置,铁素体核心明显呈不均匀分布。 本文还结合蒙特卡罗晶粒粗化模型,分析了单个铁素体晶粒在相变过程中的长大行为及其对铁素体晶粒尺寸的影响。模拟结果表明,单个铁素体晶粒在相变过程中表现为6种长大模式:抛物线状长大、延迟形核长大、暂时收缩长大、部分收缩长大、完全收缩长大以及相变后期加速长大。前三种模式证实了同步加速器3DXRD实验的测量结果,后三种模式则是本次模拟的新发现。不同铁素体晶粒周围的碳浓度场的重叠(软接触)以及铁素体晶粒之间的曲率驱动的粗化作用(硬接触)成为影响长大行为的主要因素。深入分析表明,铁素体晶粒粗化过程会明显削弱传统形变-相变耦合技术对晶粒的细化作用。 本文建立了介观尺度的同步耦合模型以描述塑性形变和组织演变同时进行的动态组织演变。并利用此模型模拟了Ae3温度之上的形变诱导铁素体动态相变。其中主要讨论了形变参数(形变量、形变温度和形变速率)对动态相变过程中组织演变以及应力-应变曲线的影响。结果表明,奥氏体的动态再结晶是影响铁素体析出的一个非常重要的因素。连续形变下的奥氏体动态再结晶、奥氏体-铁素体相变以及铁素体-奥氏体的逆相变三者之间的交互作用会导致动力学上的震荡行为。此外,低形变温度或者形变后的高冷却速度都有利于诱导铁素体的稳定性。
Other AbstractThe new generation steels have got more attentions due to the requirement of economic development. One of the important researches is enhancing the strength and the ductility using the cost-effective technology. The refinement of microstructures is one of the main approaches to enhance the strength and the ductility. For the ferrite-pearlite steels, the coupling between the deformation and the austenite-ferrite transformation is a useful method of grain refinement. Therefore, it is very important to develop a quantitative relationship between the processing of steels and their microstructure in order to produce tailor-made material by designing the composition and controlling the processing. In this dissertation, the mesoscopic models of the coupling processes between the deformation and the austenite-ferrite transformation in low carbon steels are built. The crystal plasticity finite element model is built firstly for modeling the hot deformation of austenite at mesoscale. The simulation results show that the space distribution of the mechanical behaviors of plastic deformation is non-uniform. The orientation relationship of neighboring grains can affect this distribution intensively. The further analysis shows that this non-uniform distribution results from the interaction of the active slip systems. The other result induced by plastic deformation is the increasing austenite grain boundary areas with the increasing strain. Commonly, the increasing rate of the grain boundary areas simulated by crystal plasticity finite element model is larger than that by uniform-deformed geometry models. For simulating the influences of austenite deformation on the subsequent austenite-ferrite transformation, a sequential integration model is built based on the crystal plasticity finite element model and the Monte Carlo model of phase transformation. The simulation result shows that the plastic deformation of austenite can obviously accelerate the kinetics of the subsequent austenite-ferrite transformation. Both the long-range diffusion of carbon diffusion and the short-range diffusion of Fe lattice at phase interfaces can be accelerated by the deformation. The simulated microstructure evolution indicates that the plastic deformation can markedly increase the ferrite nucleation density. It should attribute to three reasons: (1) the increased austenite grain boundary area due to the deformation, (2) the increased ferrite nuclei number per unit area at austenite grain boundaries, (3) the formation of high stored energy regions at austenite grain interiors induced by deformation. The spatial distribution of the ferrite grains is inhomogeneous because of the heterogeneous distribution of the stored energy. The growth behavior of the individual ferrite and its effects on the ferrite grain sizes are investigated by coupling a Monte Carlo coarsening model. The six classes of grain growth behavior, i.e. parabolic growth, delayed nucleation and growth, temporary shrinkage, partial shrinkage, complete shrinkage and accelerated growth, can be found during the austenite decomposition. The former three growth behaviors are in accord with the observation by 3DXRD experiments while the latter three growth modes are not observed in literatures. The complex behaviors of ferrite growth result from the overlapping of carbon diffusion (soft impingement) around the ferrite grains and the coarsening of neighboring ferrite grains (hard impingement) due to the grain boundary curvature. For the traditional thermomechanical processing in low carbon steels, the coarsening of ferrite grains during the early stage of transformation might attenuate the effectiveness on grain refinement. For investigating the dynamic microstructure evolutions (i.e. the deformation and the microstructure evolution happen simultaneously), a mesoscopic synchronous integration model is developed. The deformation induced dynamic transformation (DIDT) of a Fe-C alloy above Ae3 temperature based on this model. The influence of deformation parameters, including temperature and strain rate, on the microstructure evolution and the stress-strain curves are discussed. The simulation results show that the competition between the DRX of austenite and austenite-to-ferrite transformation causes the different microstructures and changes the shape of the stress-strain curves for the different deformation parameters. As a result of this competition, the ferrite fraction is found to oscillate during the DIDT. The stability of the induced ferrite fraction affected by the ferrite-to-austenite reverse transformation is assessed by simulating its kinetics during the isothermal holding after deformation.
Pages147
Language中文
Document Type学位论文
Identifierhttp://ir.imr.ac.cn/handle/321006/17050
Collection中国科学院金属研究所
Recommended Citation
GB/T 7714
肖纳敏. 低碳钢形变-相变耦合过程的介观模拟计算[D]. 金属研究所. 中国科学院金属研究所,2008.
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