Mechanism-based microstructure design of low-carbon giga NANOBAIN steels

• Applicants: Dr.-Ing. Daniel Schneider; Professorin Dr.-Ing. Wenwen Song
• Subject Area: Thermodynamics and Kinetics as well as Properties of Phases and Microstructure of Materials ; Mechanical Properties of Metallic Materials and their Microstructural Origins
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 490856143

This project aims at developing low-carbon NANOBAIN steels with strength at gigapascal level based on the “bottom-up” microstructure design strategy to overcome the mechanical trade-off of advanced high strength steels. The targeted bainite-austenite duplex microstructure exhibits the features of high fraction of nano-sized carbide-free bainite, film-like austenite, and a large number of interfaces. To achieve the desired refinement of bainitic microstructure with tailored mechanical properties and within a low-carbon regime, a chemical boundary concept is proposed and a flash-austenitization is designed to introduce the chemical boundary into the materials. The main research focus of the proposed work is to study the chemo-mechanically coupled effects on bainite nucleation and growth by multi-scale characterization and phase-field simulation approach. The characterization down to near-atomic scale using atom probe tomography (APT) enables the experimental observation of Mn chemical boundary for bainitic microstructure sub-refinement. The dynamic monitoring of phase transformation by synchrotron X-Ray diffraction, in-situ TEM and APT enable exploring the influences of chemical gradients and plastic relaxation on bainite nucleation and growth. The following sub-targets are aimed to be achieved: 1) Investigate the chemical-driving influence of composition inhomogeneity, elemental partitioning, chemical gradients on bainite nucleation/growth by experimental approaches (Chemo-aspect); 2) Investigate the mechanical-driving influence of matrix dislocation, plastic relaxations on bainite nucleation/growth by experiment approaches (Mechanical-aspect); 3) Formulate the thermodynamically consistent chemo-mechanically coupled variational model validated by experimental characterization for the prediction of bainitic sub-unit evolution; 4) Upscaling the chemo-mechanically coupled model to predict the bainitic sheave formation; 5) Comprehensive understanding of bainite transformation controlling strategy and produce low-carbon NANOBAIN giga steels with enhanced mechanical properties.Finally, the underlying chemo-mechanical mechanisms that govern the bainitic phase transformation will be revealed and a heat treatment control guideline of low-carbon giga NANOBAIN steels with tailored microstructure and mechanical properties will be provided. With the advantages in lean carbon composition and non-complicated production process, NANOBAIN steels are expected to contribute to the development of economic, green and sustainable manufacturing, for example contributing to the development of sustainable manufacturing of high-performance components in the automotive industry.

• DFG Programme: Research Grants