Improving the fracture resistance of forging medium manganese steel under static, dynamic and cyclic loading conditions by means of fine-dispersed, partitioning-stabilized retained austenite

用于先进锻件的现代钢要求在静态、动态和循环载荷条件下具有高强度和断裂韧性/抗力的组合。此外，它们需要易于加工，并需要良好的淬透性，以确保整个锻造过程中稳定的组织和机械性能。到目前为止，还没有可以在可接受的生产成本下有效结合上述力学性能的钢加工概念。目前用于锻件的最先进材料是中锰钢，RA属于第三代先进高强度钢。具有足够稳定性和形态均匀性的RA通过确保局部塑性来防止裂纹的形成，而塑性变形形成的马氏体则有助于阻止可能的微裂纹的扩展。该项目的概念是基于使用淬火和隔断(Q&P)热处理。获得低碳马氏体和富碳残余奥氏体相的组织，将保证良好的力学性能。将在一种新的方法中考虑残余奥氏体的作用，这与在Q&P薄板中的情况完全不同。在拟议的研究中，将探索中锰锻钢在淬火和分配过程中的组织演变的新概念，并定义各自的力学性能。这项研究旨在开发第三代先进的高强度Q&P钢，这些钢是首次通过利用热加工热直接空气淬火的节能锻造工艺生产的高疲劳强度和疲劳损伤容限钢。该项目将解决以下基本的科学和研究问题：(I)确保细小弥散的残余奥氏体的形态均匀；(Ii)确定强化机制，特别是在冲击和循环载荷条件下应变诱发马氏体相变的动力学。加载类型和相变之间的定量关系尚未定义；(Iii)残余奥氏体形态和马氏体基对断裂机制和裂纹扩展行为影响的表征。试验将包括热力学计算、膨胀试验、使用Gleeble模拟器的热力学模拟、使用包括EBSD/EBSD 3D、APT、X射线衍射和透射电子显微镜在内的各种分辨率的研究技术的综合微观结构试验，以及具有开裂机制特征的疲劳强度试验。包括有限元方法和平均场模型在内的多尺度模型将被用来模拟在静态、动态和循环加载条件下应用/试验的中锰Q&P钢热变形过程中的组织关系。

Modern steels used for advanced forgings require a combination of high strength and fracture toughness/resistance under static, dynamic and cyclic load conditions. Moreover, they need to be easy machined, and an excellent hardenability is required to guarantee stable microstructures and mechanical properties throughout the whole forging. Up to now, there are no steel processing concepts available that effectively combine the above-mentioned mechanical properties at an acceptable production cost. The most advanced material to be used for forgings are currently medium-Mn steels with RA belonging to the 3rd generation of Advanced High Strength Steels. The RA of adequate stability and morphological homogeneity prevents the formation of cracks by ensuring local plasticity, while the martensite formed as a result of plastic deformation contributes to blocking the propagation of possible microcracks. The concept of the project is based on the use of Quenching and Partitioning (Q&P) heat treatment. The obtained structure of low-carbon martensite and carbon-enriched retained austenite, will ensure favorable mechanical properties. The role of retained austenite will be considered in a novel approach, which is completely different than in the case of Q&P sheet steels. In the proposed research, the new concept of medium-Mn forging steels shall be explored with respect to microstructure evolution during quenching and partitioning and defining the respective mechanical properties. The proposed research aims at the development of 3rd generation advanced high strength Q&P steels tailored for high fatigue strength and fatigue damage tolerance that are first time produced by an energy efficient forging process applying direct air-quenching from the hot-working heat. The following fundamental scientific and research issues will be resolved in the project: (i) ensuring morphological homogeneity of fine-dispersed retained austenite; (ii) determining the strengthening mechanism, and in particular the kinetics of strain-induced martensitic transformation under impact and cyclic loads conditions. No quantitative dependences between a type of loading and the transformation have been defined yet; (iii) characterization of the effect of retained austenite morphology and martensite matrix on the fracture mechanism and crack propagation behavior. Tests will be carried out including thermodynamic calculations, dilatometric tests, thermomechanical simulations using Gleeble simulator, comprehensive microstructure tests using research techniques of various resolutions including EBSD / EBSD 3D; APT; XRD and TEM and fatigue strength tests with the characteristics of the cracking mechanism. Multiscale modelling including a finite element method approach coupled with mean-field models will be used to simulate the process-microstructure relationship during the hot deformation of medium-Mn Q&P steels to be applied/tested under static, dynamic, and cyclic loading conditions.