Carbide-free bainitic (CFB) steels have gained increasing attention in recent years because of their excellent mechanical properties. The excellent combination of strength, ductility and toughness achieved in these steels is only matched by that of Maraging steels which cost 10 to 100 more than the carbide-free bainitic steels. The excellent mechanical behavior of CFB steel is mainly due its complex microstructure (bainitic ferrite, retained austenite and martensite) consisting of a high strength phase (ultra fine bainitic ferrite) and TRIP effect from retained austenite. Carbide formation is avoided due to high silicon content which suppresses cementite precipitation from austenite. The effect of bainitic transformation time on the microstructure and mechanical properties was investigated in a steel containing 0.4%C-2.8%Mn-1.8%Si. The microstructure was characterized using optical and transmission electron microscopy; it consisted of bainitic ferrite, martensite and retained austenite. This microstructure exhibited an extended elasto-plastic transition leading to very high initial work hardening rates. The work-hardening behavior was investigated in detail using strain-path reversals to measure the back-stresses. These measurements point to a kinematic hardening due to the mechanical contrast between the microstructural constituents. The strain aging effect at room temperature on the CFB steel was also been analyzed in great detail. The static strain aging effect at room temperature can not be overlooked in the carbide free bainitic steel. After isothermal bainite heat treatment, the yield strength of the material is increased by about 80MPa, and the ultimate tensile strength is improved by more than 100MPa after aging at room temperature for one week. This phenomenum could be related to the interactions between carbon atoms and the dislocations, grain boundaries and the redisual stresses. Examination of the fracture surfaces indicated that the prior austenite grain boundaries play an important role in the fracture process. A set of experiments were designed to study the effect of ausforming on the microstructure and mechanical properties of CFB steels. Based on its mechanical behavior under tensile tests and microstructural analysis by EBSD, the TRIP effect was contributing to the work hardening behavior. The changes in morphology and variant selection of the bainitic ferrite lath in the ausformed carbide free bainitic steel were also observed. A new set of chemistry was design with reduced carbon and manganese content to further improve the weldability and the reproducibility of the carbide free bainitic steel. / Thesis / Doctor of Philosophy (PhD)

It is well known that within materials science and engineering, the advancement of steels is subject to the conflicting objectives of achieving high strength, energy absorption, and ductility within a single material. Multilayer metal composites (MLMCs), combining multiple advanced high strength steels (AHSSs), are promising candidates for designing materials that can achieve these mechanical property combinations which are unattainable by monolithic steels. However, the mechanical behaviour and corresponding properties of MLMCs are challenging to predict, due to the number of variables within the design space of the composite. Variables such as alloy design, number, thickness, configuration of layers, and interfacial bonding strength, all impact the potential mechanical properties. Accordingly, this work addressed the fracture behaviour of a multilayer AHSS composite, consisting of carbide-free bainitic (CFB) steel and high Mn twinning-induced plasticity (TWIP) steel, in both sequential deformation and co-deformation of layers to determine the potential advantages of a multilayer structure. In tensile deformation, a balanced combination of high strength (ultimate tensile strength (UTS) of 1290 MPa) and high ductility (total elongation (TE) of 23%) was achieved with a sandwich structure configuration consisting of two outer layers of the TWIP steel and an inner core layer of the CFB steel. The composite consisted of equal volume fractions of each constituent steel. The TE achieved by this structure exceeds that which previous studies would predict, which suggest that the elongation of a composite is controlled by the elongation limits of the monolithic hard layer (which in the case of the CFB steel is 13%). In the sandwich configuration, the soft outer layers contributed to increased ductility of the composite by inhibiting crack formation in the hard layer and exerting a compressive stress on the inner CFB core. The increased compression caused the CFB to yield at a lower stress (than it would in monolithic conditions), allowing it to plastically deform further, and the composite to have a greater total elongation. This was attributed to the strong interfacial bond, which enabled the layers to co-deform without any delamination. A bilayer composite consisting of the same volume fractions (as the sandwich configuration), demonstrated the same UTS, but a total elongation of 13%. The reduced ductility is a result of smaller compressive forces on the CFB, as well as, crack formation in the CFB at the 13% elongation (the TE of monolithic CFB), which led to immediate fracture of the sample. In tensile deformation with a pre-existing crack (double-edge notched tension (DENT)), the bilayer composite exhibited a high essential work of fracture (EWF)/cracking resistance. In the sandwich configuration, the outer TWIP layers exerted a compressive stress on the inner CFB core, which was possible due to the strong interfacial bond. This compressive stress and the thin layer configuration caused the CFB core to fracture in a ductile manner. The impact energy absorption of the sample was investigated by Charpy impact testing, and the procedure of crack propagation analyzed by three-point bending. High energy absorption was achieved with a notch positioned in the TWIP layer, in which the composite exceeded the energy absorption of either monolithic steel. The sample absorbed the energy through plastic deformation of the two layers, as the interface prevented crack formation in the CFB layer. When the notch was positioned in the CFB layer, the impact energy absorption was nearly equal to that of the monolithic TWIP steel. In this configuration, the composite absorbed the energy through dissipation of the propagating crack along the interface, causing delamination and subsequent bending of the TWIP layer. In assessing the experimental results in this work, it was determined that in both deformation conditions (sequential and co-deformation), the composite is sensitive to the layer configuration. To produce an optimal and balanced combination of mechanical properties (strength, energy absorption, and ductility), it is critical to inhibit or at minimum, delay crack initiation within the CFB (hard steel) layer. Overall, this research shows that the experimental multilayer composite is promising for developing an AHSS structure that can demonstrate properties unattainable by monolithic steels. / Thesis / Master of Applied Science (MASc) / Advanced high strength steels are generally limited by competing mechanical properties of strength and impact energy absorption. Combining hard and soft phase microstructures within one material (i.e. dual-phase steel) thermodynamically restricts the material by the composition and the possible heat treatment conditions. It also leads to large strain gradients resulting in void formation and failure. Instead, multilayer composites can be designed with each layer independently exhibiting a monolithic microstructure that optimizes each desired mechanical property. The bonding strength between the layers can also be adjusted, altering the distribution of stresses when the material is deformed. This research aimed to analyze a multilayer metal composite that combined a soft-phase austenitic steel exhibiting high energy absorption with a hard-phase carbide-free bainitic steel exhibiting high strength. The material was evaluated in two conditions: i) under co-deformation where the layered structure was deformed parallel to the interface and ii) under sequential deformation, where stress was applied to one layer at a time. The results indicated that in both conditions, the composite was sensitive to the configuration of the layers. It demonstrated the potential to exhibit a combination of high strength and high energy absorption capabilities in sequential deformation. In co-deformation, certain configurations of the composite were able to exhibit increased ductility and fracture resistance (improved from the monolithic hard steel). In both cases, the critical design factor was that crack initiation and propagation must be restricted in the hard material to achieve balanced mechanical properties of strength and energy absorption.