Development of low carbon Nano bainitic steels with balanced strength and ductility

Asmaa Elbeltagy, Xiao Shen and Wenwen Song

Steel Institute, RWTH Aachen University, Intzestr.1, 52072 Aachen, Germany

Bainitic steels that contain bainite-austenite sandwich-like structure stands out as a promising candidate for solving the ductility-strength trade-off dilemma for the automotive market, especially at lower carbon and manganese contents with enhanced weldability. However, the presence of the blocky austenite with low austenite stabilizing elements leads to degrading the work hardening rate. The study aims to design and optimize the chemical composition and the processing route to achieve a combination of 1180 MPa ultimate tensile strength with total elongation of 25% with lean alloy concept. Nano-engineering approaches such as cyclic thermal treatment, Ausforming, Nano-precipitates and dislocation engineering, are employed aiming at replacing the blocky austenite islands with a triplex nano-structured austenite, bainite with minimal martensite fraction while stabilizing the austenite by introducing high defects density and enhanced carbon diffusion paths to achieve two or three-fold deformation mechanisms; TRIP, TWIP, and dislocation multiplication. The chemical compositions and the heat treatments are designed using ThermoCalc and MatCal, time-temperature transformation (TTT), and continuous cooling transformation (CCT) diagrams. The chemical composition design is based on the following criteria: 1) Widening the austenite stability window while limiting the ferrite formation window to the favor of bainite, 2) inducing high density of nano-carbides to act as additional nucleation sites and suppress grain growth by using calculated amounts of high temperature and medium temperature grain refinement elements; Nb, V and, Mo, and 3) suppressing cementite and cementite-like precipitates. Multi-scale characterization techniques, e.g. electron backscattering diffraction (EBSD), transmission electron microscopy (TEM) and atom probe tomography (APT) are used to determine and optimize the favourable processing route and to understand the microstructure-property relationship.