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低碳钢热变形中微观组织演变的元胞自动机模拟
Alternative TitleSimulation of the microstructure evolution during hot deformation in low carbon steels using a Cellular Automaton approach
郑成武
Subtype博士
Thesis Advisor李依依
2009-05-22
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奥氏体/铁素体 热变形 动态相变 再结晶 晶体塑性有限元 元胞自动机
Abstract钢铁材料因其良好的性能和可回收性而成为现代生活中最主要的工程结构材料,钢材产品的力学性能与材料的组织状态密切相关。再结晶和奥氏体—铁素体相变是钢铁材料制备、加工和热处理过程中发生的很普遍的物理冶金现象,通过钢材在高温变形中发生的奥氏体再结晶及奥氏体—铁素体相变可以细化晶粒和改善组织,使产品达到优良的综合性能。因此,研究钢铁材料在热变形中发生的微观组织转变及动力学,对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。本文从介观尺度计算模拟的角度来研究低碳钢热变形中发生的奥氏体再结晶及奥氏体—铁素体相变。 本文首先利用晶体塑性有限元和元胞自动机耦合的方法对低碳钢热变形后的奥氏体静态再结晶进行了模拟。为了描述预变形对随后奥氏体静态再结晶微观组织转变和动力学的影响,利用晶体塑性有限元方法在晶粒尺度上模拟了奥氏体高温变形后的变形组织及形变储能分布,然后将这些结果映射到元胞自动机网格上作为随后奥氏体再结晶模拟的初始条件。利用该耦合模型着重研究了奥氏体不均匀变形对再结晶组织演变动力学的影响,同时对不同初始变形量下的再结晶过程进行了模拟。结果表明,奥氏体在经历塑性变形时,其力学响应的空间分布很不均匀,这种变形的不均匀性会导致奥氏体再结晶核心呈现出明显的不均匀分布,再结晶晶核在形变储能高的位置优先集中形核,同时再结晶动力学明显与经典JMAK理论背离。 为了研究碳钢热变形过程中发生的奥氏体—铁素体相变行为,本文建立了描述塑性变形与组织演变同时进行的同步耦合元胞自动机模型,模拟了低碳钢Q235动态形变诱导铁素体相变。通过与奥氏体的等温分解相变的模拟结果对比,在介观尺度上讨论了形变诱导铁素体相变导致铁素体晶粒“超细化”的机理;并根据模拟结果讨论了初始奥氏体晶粒尺寸及应变速率等参数对形变诱导铁素体相变组织和动力学的影响。结果表明,铁素体的“不饱和”形核和“有限”生长是形变诱导铁素体相变的两个特征,也是造成形变诱导铁素体超细化的重要原因。形变诱导相变形成了由大量细小铁素体晶粒和弥散分布在铁素体晶界/三叉晶界的少量细小片状/岛状残余奥氏体组成的双相组织,这种微观组织结构对提高低碳钢的力学性能有利。 本文尝试将介观尺度的微观组织模拟扩展应用到带钢多道次热轧过程中的微观组织预报当中。与宏观经验/半经验物理冶金模型相比,介观尺度的组织预报更能体现微观组织特征及其演变过程,不仅能够描述不同热加工制度下微观组织的平均特征 ( 如:再结晶转变分数、平均晶粒尺寸演变等 ) ,还能预报微观组织的空间分布特征及尺寸分布特征。模型充分考虑了热轧过程中各金属学现象的作用过程,以实际轧钢生产线数据作为输入参数保证模拟结果与真实结果的可比性。利用所建模型,针对热轧带钢精轧7道次的再结晶过程进行了模拟,并将模拟结果与自行开发的ROLLAN软件的预报结果进行了对比。
Other AbstractAs the most important structural materials, steels with good properties and low costs have been mostly used in modern industry. The properties of steel products are directly raltated with their final microstructures. As two kinds of the most commen physical metallurgical phemomena taking place in the processing and heat treatment of steels, recrystallization and austenite-to-ferrite transformation have been used widely in steel industry to refine the microstructure and then to improve the mechanical properties. Consequently, it is of great importantance to develop a quantitve relationship between the processing of steels and their microstructure in order to produce tailor-made materials by designing the composition and controlling the processing. In this dissertation, an alternative methodology of mesoscopic modeling is devolped to investigate the microstructural evolution during hot deformation in low carbon steels. The austenite static recrysatllization following a hot deformation is simulated by coupling a cellular automaton (CA) model with a crystal plasticity finite element model (CPFEM). With a purpose to describe the influence of pre-deformation on the microstructural evolution and transformation kinetics of austenite recrystallization, the local stored energy of deformation is firstly simulated by CPFEM. These results are then mapped onto the CA lattice as the initial states for the subsequete recrystallization simulation. The effect of the inhomogeneous distribution of the stored energy on the recrystallization kinetics is addressed using this coupling method. The simulation results reveal that the space distribution of the mechanical responsed of plastic deformation is very heterogeneous. The inhomegeous distribution of the deformation results in non-uniform nucleation with a cluster pattern as well as a remarkable deviation in the recrystallization kinetics with the classical JMAK theory. In order to investigate the austenite-to-ferrite transformation occurring in hot deformation, a synchronous intergration CA model which addresses the simultaneity between deformation and microstucture evolution is also developed. The dynamic strain-induced transformation (DSIT) in a low carbon steel is investigated by using this model. It permits the mechanisms for the refinement of the DSIT ferrite to be studied, together with the effects of some important parameters, e.g. the the prior austenite grain size (PAGS) and strain rate. The simulated results indicate that the refined ferrite grains derived from the DSIT are achieved by “unsaturated” ferrite nucleation and “limited” growth. Furthermore, ferrite dynamic recrystallization is occurring to subdivide the DSIT ferrite grains formed at the early stage of transformation and maintain their equiaxed morphology. The simulated microstructure in the DSIT is in good agreement with the quenched dual-phase microstructure observed in the optical micrograph, which consists of fine-grained ferrite and fine martensite islands/flakes dispersing in the matrix. With a purpose to validate the industrial usability of the mesoscopic microstructure modeling, an intergrated CA model is developed to predict the microstructural evolution of the austenite recrystallization during multi-pass hot rolling of steel strip. In contrast to the macroscopic empirically-based model, the mesoscopic model enables both quantitative and topographic predictions of the microstructure evolution, rather than mere average microstructural features. The use of practical operational parameters inputted for each pass ensures a realistic model capable of being tested against actual microstructural results. The results are compared with the predictions by the in house software ROLLAN and are found to be in good agreement.
Pages125
Language中文
Document Type学位论文
Identifierhttp://ir.imr.ac.cn/handle/321006/17169
Collection中国科学院金属研究所
Recommended Citation
GB/T 7714
郑成武. 低碳钢热变形中微观组织演变的元胞自动机模拟[D]. 金属研究所. 中国科学院金属研究所,2009.
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