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Abstract
Low C (<0.2 wt.%), Mo-based microalloyed steels with soft ferritic microstructures are widely used in the automobile sector due to their excellent stretch-formability. Microstructure engineering through advanced thermomechanical processing (a-TMP) is required to achieve high yield strength in these steels. After hardening and tempering, the same steels with martensitic microstructures are attractive for high-strength structural applications. While the martensitic microstructure evolution for plain and high-C microalloyed steels is well established, it remains largely unexplored for low C microalloyed steels.
This thesis focuses on alloy-design and process optimization using computational and experimental tools to develop a-TMP processes aimed at ferrite strengthening for leaner grades of microalloyed steels. Three model steels: (i) ultra-low C, NbMo [0.05C-1.43Mn-0.16Si-0.13Mo-0.04Nb], (ii) low-C, Nb [0.10C-1.45Mn-0.20Si-0.04Nb], and (iii) low-C, NbMo [0.10C-1.48Mn-0.16Si-0.23Mo-0.05Nb] (in wt. %) were designed. Their ferritic and martensitic microstructure evolutions were investigated in detail using advanced characterization techniques. Furthermore, martensitic microstructures were subjected to a unique continuous cyclic quenching heat treatment to examine their softening behavior.
An optimized a-TMP with a two-phase (γ+α) finish rolling at 750 ℃ and coiling at 670 ℃ for 1 hour was proposed resulting in a fine polygonal ferritic microstructure with a mean grain-size of 1.6 µm and NbC precipitates of ~1.9 nm in diameter in the NbMo steel. This strengthened the ferrite matrix to a proof strength of 484 and an ultimate tensile strength of 707 MPa. The hierarchical martensite microstructure of the Nb steel showed finer prior austenite grains, packets, and blocks and a higher number of lath martensite variants when compared to the NbMo steel. Lath substructures in the Nb steel consisted of dislocations, auto-tempered carbides at interfaces, and long and short twins. The NbMo steel frequently exhibited intra-lath carbide-dislocation entanglements, without any twinning. Site-specific atom probe microscopy revealed that the C concentration in auto-tempered carbides in both steels was significantly lower than expected stochiometric values. Finally, the continuous cyclic quench treatment showed a more pronounced martensite softening in the NbMo steel which was correlated to the interface evolution.
This thesis demonstrates microstructural design capabilities for low C microalloyed steels to satisfy industrial demands for advanced properties.
