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

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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.