【作者】 李杰；

【导师】 王国栋； 贾涛；

【作者基本信息】 东北大学 ， 材料加工工程， 2018， 硕士

【摘要】 各国为解决汽车能源消耗和环境污染的问题提出了汽车轻量化的设想。采用高强度钢替代传统低碳钢在车身中的应用可以在保证安全的前提下较为经济便捷地实现汽车轻量化要求,因此最近几十年高强度汽车用钢的研发工作在世界范围内大量展开,其中NanoSteel公司研发的同名合金材料纳米钢以新的合金设计思路“大量使用P区元素”和新的强化机制“静态纳米相细化机制、动态纳米相强化机制”引起了科研工作者们的注意,但受其商业专利的保护,这一材料的具体细节尚未披露,对这一材料进行探索研究,能为先进高强钢的研发提供一些新的参考和思路。本文以NanoSteel公司2012年授权专利中的典型纳米钢合金为范例,通过熔炼、热轧、热处理与拉伸性能测试等方式展开研究,目的在于揭示这一材料在热加工过程中的微观组织演变规律与强化机制。主要研究内容和结果如下:(1)采用中频感应炉熔炼、常规模铸制备10kg大尺寸铸锭,与真空电磁搅拌电弧炉熔炼的100g纽扣锭进行对比,通过光学显微镜、电子扫描显微镜的铸态组织形貌观察,结合电子探针的成分分析和电子背散射衍射技术的物相分析,研究了凝固速率对铸态组织结构的作用规律。(2)在Thermo-Calc计算的热力学平衡相图基础上,开展了实验合金的连续加热与连续冷却过程中的Dictra相变动力学模拟、相变仪实验与热轧实验验证。由于置换型合金元素在奥氏体向铁素体、铁素体向奥氏体相变中的扩散行为差异,导致铸态组织中铁素体在加热过程中转变为奥氏体,而高温奥氏体在连续冷却中稳定至室温。(3)开展了热处理过程微观组织演变的研究,尤其是热处理温度对微观相组成的影响规律。结果表明,热处理温度是通过影响置换型合金元素的扩散速率、奥氏体向铁素体相变驱动力来间接影响热处理后的微观相组成。(4)对实验合金的拉伸力学性能进行了一系列测试,通过X射线衍射的物相鉴别与透射电子显微镜的精细组织观察分析了拉伸过程中的强化行为,重点研究了奥氏体向铁素体的动态转变行为(相变诱导塑性),但并未观察到NanoSteel授权专利中所述的属于#186、#190空间群的纳米动态析出物。(5)从降低合金成本的角度出发,设计了仍保有大量P区元素的减量化实验合金,利用光学显微镜、电子显微镜以及电子探针手段分析了微观组织结构,发现硼化物与基体组织粗大,拉伸力学行为表现为脆性。更多还原

【Abstract】 In order to solve the problem of energy consumption and environmental pollution caused by automobiles,many countries have put forward the concept of automobile lightweight.The use of high-strength steel instead of the traditional low-carbon steel in automobile body structure can be more economical and convenient to achieve the weight reduction of automobiles under the premise of ensuring the safety.Therefore,in the last decades a large scale R&D works of high-strength automotive steels have been carried out around the world.Among them,NanoSteel company developed the homonymous alloy materials with a new design ideas "the addition of high content of P group elements",and new strengthening mechanisms "Static Nanophase Refinement mechanism and Dynamic Nanophase Strengthening mechanism".The new alloy has drawn the attention of researchers.However,the details have not yet been disclosed due to the protection of their commercial patents.Exploration and research on this new alloy will provide some new ideas and guidance for the development of high strength steels.In this paper,typical alloy from NanoSteel’s 2012 patent will be taken as an example.By melting the experimental alloy and observe the material microstructure in various states,as well as test the mechanical properties of the material to understand and demonstrate the specific details of NanoSteel.The main contents and research results are as follows:(1)Ten kilos of large size ingot were smelted by medium frequency induction furnace and cast in conventional die,compared with 100g button ingot melted by vacuum electromagnetic stirring electric arc furnace.The microstructure of the as-cast alloy was observed by optical microscope and scanning electron microscope.Combined with the composition analysis of electron probe and the phase analysis of electron backscatter diffraction technology,the effect of solidification rate on the as-cast microstructure was studied.(2)Based on the thermodynamic equilibrium phase diagram calculated by Thermo-Calc,Dictra was used to simulate phase transformation in the continuous heating and continuous cooling of the experimental alloy and the results were verified by the phase transition and hot-rolling experiments.Due to the difference of the diffusion behavior of the substitutional alloying elements in austenite to ferrite and ferrite to austenite transformation,the ferrite in the as-cast microstructure could transform to austenite during heating but the high temperature austenite was stable to room temperature during continuous cooling.(3)The microstructure evolution of the heat treatment process was studied,and the influence rules of the heat treatment temperature on the micro-phase composition were especially investigated.The results showed that the heat treatment temperature indirectly affects the micro-phase composition after heat treatment by influencing the diffusion rate of substitutional alloying elements and the driving force of austenite to ferrite transformation.(4)Tensile mechanical properties of the experimental alloys were tested in a series of ways.Combined with the phase identification results obtained by X-ray diffraction and the fine structure observetion by a transmission electron microscope,the strengthening behavior in the stretching process was analyzed.The dynamic transformation behavior of austenite to ferrite(Transformation-induced plasticity)was mainly studied.However,Nano-dynamic precipitates belonging to the#186 and#190 space groups described in the NanoSteel grant patent were not observed.(5)From the perspective of reducing the cost of the alloy,an alloy reduction experimental steel which still retains a large amount of P group elements was designed.The microstructure of the steel was analyzed by means of optical microscope,electron microscope and electron probe.It was found that the boride and matrix grains of the steel were coarse and the mechanical behavior was brittle.更多还原