Temperature dependent mechanical properties of as-cast steels : Experimental and theoretical studies

Sammanfattning: The temperature-dependent mechanical properties of steels are important to avoid processing defects, to understand and to improve the high-temperature performance. At the same time, having access to thermal properties gives us opportunity to assess the first-principles theoretical predictions at elevated temperatures. These properties are directly bound up with the performance of individual phase and also the evolving microstructure states at different thermalmechanical processes. In the present thesis, the temperature-dependent mechanical properties of continuously cast steels and iron are investigated using experimental and theoretical methods. Experimental studies are performed centering on the influence of thermal cycles occurring in secondary cooling.The temperature reversion in secondary cooling makes the hot ductility trough occurring at higher temperatures with greater depth. Increasing the reversion rate, the low temperature end of the ductility trough slightly extends to lower temperatures. As indicated by microstructure examinations, the intergranular fracture contributed from the thin film-like ferrite and (Fe,Mn)S particles slightly changes with the varying thermal cycles; however, the widmanstatten ferrite observed in the temperature reversion process seriously deteriorates the ductility. Due to the temperature reversion process, the peak stress slightly declines and the peak of strain to peak stress moves to higher temperatures. On the other hand, the sequential formations of ferrite and pearlite in the austenite transformation are indicated by two distinct peaks on the thermal expansion coefficient. By applying the developed concise model, the volume fractions of ferrite, pearlite, and austenite are quantitatively monitored in the phase transformation. Either increasing the cooling rate or the content of austenite stabilizing atoms Ni and Cu, the austenite transformation occurs at relatively low temperatures and indicates a greater phase transformation rate for both ferrite and pearlite. In addition, the final fraction of ferrite/pearlite increases/decreases with increasing the cooling rate, increasing the alloying atoms like Ni, Cr and Cu or lowering the carbon content.The temperature dependence of the polycrystalline Young’s modulus and the tetragonal shear modulus c0 of iron is predicted using ab initio calculations within the exact muffin-tin orbitals formalism. The dependence exhibits a good consistency with that of the peak stress observed in the experiments for the commercial steel. Despite the significant effects of magnetic sate and crystal structure on the elastic property of iron, the magneto-volume coupling primarily determines the temperature dependence for the single phase. In contrast, the dominant role of the volume expansion is observed for both the paramagnetic (PM) face centered cubic (fcc) and body centered cubic (bcc) Fe, although they show different magneto-elastic behaviors. Based on the theoretically predicted thermal expansion for PM bcc Fe, both the lattice vibrations and the magnetic evolution contribute to the thermal expansion, and the former is dominant.