基于轧制工艺调控中锰TRIP钢组织性能的实验与机理研究

This research focuses on control of microstructure and mechanical properties of medium Mn steel which acts as one of promising candidates of the 3rd generation advanced high strength steels (AHSS) for vehicles. Based on the effect of the initial microstructure on microstructural evolution during the austenite reverted transformation (ART) annealing and resulting mechanical properties for medium Mn steels, controlling the initial microstructure characteristics before continuous annealing, e.g., dislocation density, misorientation distribution of grain boundaries, stored energy distribution of cold work and positions of retained austenite by employing optimized technology of cold rolling and softening annealing will be carried out. It can enable an effective controlling method of the mechanical stability of meta-stable austenite as well as the morphology and mechanical properties of matrix, which will add techniques to further increase the product of strength and elongation (PSE) of medium Mn steels. In this project, experimental test and numerical simulation by cellular automaton (CA) modeling will be combined to perform the research on medium Mn TRIP steels containing Al element for continuous annealing process. The study is aimed at revealing the effect of initial microstructure characteristics on nucleation and growth of reversed austenite and recrystallized ferrite grains, developing a CA model for the precise description of microstructure evolution, and further clarifying the interaction between reverted transformation of austenite and mechanism of recovery and recrystallization for deformed microstructure. On the basis of the abovementioned research, the theoretical foundation of controlling microstructure and mechanical properties of medium Mn steel utilizing the cold rolling parameters will be established, which provides theoretical reasons for optimizing rolling technology, and finally the trial-manufactured cold rolling medium Mn steels with the PSE value greater than 45 GPa% will be obtained.

针对第三代汽车用先进高强钢中的中锰钢组织性能控制开展研究，利用初始组织状态对中锰钢逆相变退火过程中的组织演变及最终的力学性能影响作用，通过优化冷轧变形和软化退火工艺来调控连续退火前中锰钢初始组织的位错密度、晶界取向差、变形储能及残余奥氏体分布等，从而掌控冷轧中锰钢中亚稳奥氏体的力学稳定性以及基体组织的形貌和力学特性，为进一步提高中锰钢的强塑积增加技术手段。本项目以连续退火生产工艺的含Al中锰TRIP钢为研究对象，拟采取实验研究和元胞自动机模拟相结合的方法，揭示初始组织状态对奥氏体逆相变和变形组织再结晶形核、长大的影响规律；构建可准确预测显微组织演变过程的元胞自动机模型；阐明奥氏体逆相变与变形组织回复、再结晶的相互作用规律；建立利用轧制参量调控中锰钢组织性能的理论基础，为优化轧制工艺提供理论依据，以此为指导试制出强塑积超过45 GPa%的冷轧中锰钢样件。

本课题以汽车轻量化为研究背景，以第三代先进高强钢中最具发展前景的高强塑积中锰钢为研究对象，主要研究了合金元素、冷轧压下率和退火工艺等对实验钢显微组织和力学性能的影响，着重探讨了中锰钢的变形行为和强韧化的影响因素。目的在于揭示显微组织和力学性能两者之间的内在联系及强韧化机理，为高强塑积中锰钢的开发和组织性能的控制提供理论指导，进而开发兼具低成本、高性能的中锰钢。经过三年的努力，主要研究目标均已实现，取得的重要研究结果如下：. 1)冷轧压下率不仅影响变形基体退火过程中的再结晶程度及组织形貌，还影响亚稳奥氏体晶粒尺寸和形貌；冷轧变形量越大，退火时再结晶越充分，拉伸变形时Lüders应变越显著。2)粗大的纤维状δ-铁素体晶粒作为软相，能够提高中锰钢塑性；而在塑性变形初期亚稳奥氏体发生的TRIP效应，引起局部硬化，能够起到抑制Lüders应变的作用。3)对于Fe-8.9Mn-1.54Si-1.0Al-0.056C合金，提高的Mn含量能够起到抑制再结晶的作用，再配合短时间的退火，可使退火后仍保留一定量的变形态组织，提高位错密度，最终可实现高屈强比且连续屈服的设计目标。4)基于Matlab语言，根据再结晶理论、奥氏体逆相变原理及扩散原理，建立了冷轧态中锰钢退火过程中的微观组织演变的元胞自动机（CA）模型。利用建立的CA模型，可以实现对中锰钢热处理过程中显微组织演变全过程模拟，以及获得退火后最终显微组织特征，如再结晶分数、奥氏体含量、晶粒尺寸及合金成分等。5)Fe-8.9Mn-1.59Si-1.0Al-0.25C合金的强塑积最高可以达到57.7GPa%，其中抗拉强度和延伸率分别为1405MPa和41.0%，实现了高强塑积的设计目标，并且具有连续屈服特征。6)Fe-8.9Mn-1.59Si-1.0Al-0.25C合金之所以在具有高强度的同时仍然兼具优异的塑性，主要是因为亚稳奥氏体在变形过程中发生了TRIP和TWIP效应的协同强韧化。. 上述研究成果不仅为汽车工业提供了力学性能优异的汽车用钢新品种，而且还大大丰富了现有物理冶金学知识，为钢铁材料，特别是中锰钢的显微组织工程设计和力学性能优化提供了新思路和新方法，同时具有应用价值和科学意义。