高强韧多相钢工艺、组织、性能及相互关系的物理模拟

发布时间：2018-07-08 20:46

本文选题：冷轧 + TRIP钢　；参考：《北京科技大学》2015年博士论文

【摘要】：本文利用原位高能X射线衍射(HEXRD)技术,并结合光学显微镜(OM)、扫描电镜(SEM)、透射电镜(TEM)等分析手段,对低合金冷轧C-Mn-Al-Si系TRIP钢的组织-性能关系进行了研究,重点探讨了其微观变形机理,并建立了能够反映多相材料变形过程中各相间应力应变配分行为的本构模型；同时利用Thermo-Calc和DICTRA等热力学计算软件对低合金冷轧TRIP钢工艺-组织的对应关系进行了有益的探索。结果如下：首先对两种不同C含量的低合金冷轧C-Mn-Al-Si系TRIP钢两相区退火后,400℃保温不同时间所得组织及其力学行为进行了研究,结果显示：相同的贝氏体区等温时间下,与低C含量的钢相比,C含量较高的钢获得了相对较少的贝氏体和较多的马氏体,并且表现出较高的屈服强度和抗拉强度；与此同时,较高C含量的钢组织中较高的残余奥氏体含量及其C含量使该钢的组织获得了更高的延伸率,从而体现出了较好的综合力学性能。随着贝氏体区等温时间的延长,两种钢中的贝氏体含量不断增加而马氏体含量不断减少；屈服强度逐渐提高而抗拉强度逐渐下降,延伸率则总体呈现增加的趋势。反映实验钢强度与塑性匹配情况的强塑积值线性依赖于组织中残余奥氏体的含量与其C含量的乘积值。实验钢拉伸变形过程中应变诱导马氏体相的生成速率r与增量应变硬化指数nincr.随应变的变化曲线具有很好的对应关系,体现了TRIP效应对实验钢变形过程中组织加工硬化能力的决定作用。基于同步辐射的原位HEXRD实验揭示了实验钢变形过程中各组成相之间存在的应力配分行为,以及各相的屈服强度、卸载后的残余应力、残余奥氏体的转变动力学等结果。多相钢弹性变形阶段由于组织中各相弹性模量较为接近,应力的配分不是十分明显；塑性变形开始后由于组成相流变行为的差异,造成各相承担的载荷发生了重新分配,铁素体相作为较软的相承担了较小的载荷,但是表现出较大的塑性应变；而贝氏体、残余奥氏体、马氏体相在变形过程中承担了相对较大的应力,但是它们对塑性的贡献相对较小,扮演着“硬相”的角色。另外,研究发现组织中亚稳的残余奥氏体相在变形过程中的转变动力学对各相间的协调变形行为或应力配分具有显著的影响,继而影响了材料最终的力学行为。基于Gladman型的混合定则以及原位HEXRD实验结果,并结合描述单相材料力学行为的基于位错演化理论的M-K模型和反映残余奥氏体变形过程中应变诱导马氏体相变转变动力学的O-C模型,建立了低合金TRIP钢应力-应变关系的本构方程。模型不仅考虑了TRIP效应,同时将对材料力学行为同样具有重要影响的复合材料效应也考虑在内,即各相间的协调变形行为对材料宏观力学行为的影响。模型中各参量均具有明确的物理含义,特别是Gladman型混合定则中的指数n,其值变化的情况反映了多相材料变形过程中各组成相之间的协调变形行为。n随应变一般呈现先降后增的趋势。变形初期,塑性变形主要集中在“软相”上,造成组织中“硬相”和“软相”之间应变不相容程度逐渐增加,对应了n值的下降；应变继续增加,“硬相”屈服并发生塑性变形,同时加之“软相”中位错动态回复的进行,使得它们之间应变不相容程度有所缓和,对应了n值的上升。与此同时,发现n值还受到残余奥氏体转变动力学的显著影响,也就是残余奥氏体相的应变诱导马氏体相变将会对各相间应力应变的配分行为产生影响。鉴于此,建立了本研究所用成分的实验钢中残余奥氏体相的应变诱导马氏体的生成速率r与指数n之间的定量关系,从而为模型的实际应用奠定了基础。利用热力学计算软件对低合金冷轧TRIP钢热处理过程中的铁素体／奥氏体临界区退火和随后的贝氏体区保温的过程进行了模拟计算,建立起了关于低合金冷轧TRIP钢工艺-组织关系的预测方法,并通过实验对此方法的可行性进行了验证。利用该方法可以对低合金冷轧TRIP钢实际工业生产中工艺参数的确定提供有意义的指导。
[Abstract]:In this paper, in situ high energy X ray diffraction (HEXRD), optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and other analytical methods are used to study the relationship between microstructure and properties of low alloy cold rolled C-Mn-Al-Si system TRIP steel. The mechanism of micro deformation is discussed, and the deformation process of multiphase materials can be reflected in each process. At the same time, the constitutive model of stress and strain distribution in phase is made, and the corresponding relationship between process and structure of low alloy cold rolled TRIP steel is explored by using Thermo-Calc and DICTRA. The results are as follows:
At first, the microstructure and mechanical behavior of two low alloy cold rolled C-Mn-Al-Si TRIP steel TRIP steels at different time after annealing at different time were studied. The results showed that, under the same bainite isothermal time, compared with low C content steel, the steel with higher C content obtained relatively less bainite and more. At the same time, the higher residual austenite content and C content in the steel structure with high C content make the microstructure of the steel get higher elongation, which shows better comprehensive mechanical properties. With the prolongation of bainite isothermal time, two kinds of steel in the steel. The content of martensite is constantly increasing and the martensite content decreases continuously, the yield strength increases gradually and the tensile strength decreases gradually, and the elongation is generally increased. The strong plastic value reflecting the strength and plasticity of the experimental steel is linearly dependent on the product value of the content of retained austenite and the content of C in the tissue. The formation rate of strain induced martensite phase R and the increment strain hardening exponent nincr. have a good correspondence with the change curve of strain, which reflects the determining effect of TRIP effect on the working hardening ability of the experimental steel during the deformation process.
The in-situ HEXRD experiment based on synchrotron radiation reveals the stress distribution between the phases of the experimental steel, the yield strength of each phase, the residual stress after unloading, the transformation kinetics of the retained austenite, and so on. The elastic deformation phase of the multiphase steel is close to the elastic modulus of each phase and the distribution of the stress. It is not quite obvious; the load of each phase is redistributed due to the difference of the rheological behavior of the composition phase after the plastic deformation begins. The ferrite phase acts as a softer phase with a smaller load, but shows a large plastic strain, and the bainite, the remnant austenite and martensite phase bear relative in the process of deformation. Large stress, but their contribution to the plasticity is relatively small and plays a "hard phase" role. In addition, it is found that the transition kinetics of metastable retained austenite phase in the process of the deformation of the tissue has a significant effect on the coordinated deformation behavior or the stress distribution in each phase, and then influences the ultimate mechanical behavior of the material.
Based on the Gladman type mixed rule and in situ HEXRD experimental results, the constitutive equation of the stress strain relationship of the low alloy TRIP steel is established by combining the M-K model describing the mechanical behavior of the single-phase material based on the dislocation evolution theory and the O-C model reflecting the transformation kinetics of the strain induced martensitic transformation during the residual austenite deformation. It takes into account not only the TRIP effect but also the effect of composite material which has an important influence on the mechanical behavior of the material, that is, the influence of the coordinated deformation behavior between each phase on the macroscopic mechanical behavior of the material. All the parameters in the model have a clear physical meaning, especially the index n in the Gladman type mixing rule, and the change of its value. The situation reflects the coordinated deformation behavior between the components of various phases during the deformation process of multiphase material.N, the trend of the strain first descends and then increases with the strain. At the beginning of the deformation, the plastic deformation is mainly concentrated on the "soft phase", which causes the strain incompatibility between the "hard phase" and "soft phase" in the tissue to increase gradually, corresponding to the decrease of the n value; strain continues to continue. In addition, the yield and plastic deformation of "hard phase", and the dynamic recovery of dislocation in the "soft phase", make the strain incompatibility between them moderate and corresponding to the rise of the n value. At the same time, it is found that the n value is also influenced by the kinetics of the retained austenite transformation, which is the strain induction of the retained austenite phase. Martensitic transformation will affect the distribution of stress and strain between each phase. In view of this, the quantitative relationship between the formation rate of strain induced martensite R and the exponential n in the residual austenite phase of the experimental steel used in this study is established, thus laying a foundation for the practical application of the model.
The thermodynamic calculation software was used to simulate the annealing process of the ferrite / austenite critical zone and the subsequent bainite heat preservation during the heat treatment of low alloy cold rolled TRIP steel. The prediction method of the relationship between the process and the structure of the low alloy cold rolled TRIP steel was established, and the feasibility of this method was verified through the experiment. This method can provide meaningful guidance for the determination of technological parameters in the actual industrial production of low alloy cold rolled TRIP steel.
【学位授予单位】：北京科技大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：TG142.1

【参考文献】