Development of Advanced High Modulus Steels for Automotive Applications

The increasing demand for high-modulus steels in automotive applications is driven by the need for improved mechanical performance, such as strength and stiffness while maintaining lightweight properties for fuel efficiency. This thesis evaluates the potential of two processing techniques, vacuum induction melting (VIM) and field-assisted sintering technology (FAST), for the development of high-modulus steels through the incorporation of titanium diboride (TiB2) as a ceramic reinforcement in a microalloyed steel (MA) matrix. The MA steel was first fabricated through VIM and subjected to thermomechanical processing (TMP), including hot rolling and plane strain compression (PSC), to refine the alloy and study phase transformations under different cooling rates. Through electron backscattered diffraction (EBSD), confirmation of prior austenite grain boundaries (PAGBs) was found in MA samples quenched from recrystallisation-stop and limit temperatures. MA steel cooled at 0.1˚C/s exhibited a pro-eutectoid phase and pearlite. The introduction of TiB2 into the MA steel matrix, in 5 and 7.5% volume fractions, demonstrated the positive effects of grain pinning mechanisms and martensitic transformation at fast cooling rates (30˚C/s). At slower cooling rates (0.1˚C/s), differences between the 5MASC and 7.5MASC phase transformations were observed, though the grain pinning effects of TiB2 were evident. Tensile testing and resonant frequency damping analysis (RFDA) showed a significant improvement in stiffness, with Young’s modulus increasing from 208 GPa for MA steel to 239 GPa for 7.5MASC. FAST was also explored as a novel powder metallurgy route to manufacture high-modulus steels. Using S4140 as a baseline material, the FAST process demonstrated its ability to consolidate powder effectively, although additions of TiB2 impeded full diffusion bonding, resulting in incomplete densification and brittle fracture behaviour in the MASC consolidated sample. Microscopy analysis confirmed the role of TiB2 as a diffusion barrier. FAST demonstrated improved densification and diffusion bonding with increasing dwell time, temperature, and pressure but issues of phase inhomogeneity in larger samples and porosity were observed.