No abstract

High Young's modulus steels can be fabricated based on the concept of metal matrix composites. In this paper, a number of reinforcing ceramic phases with high Young's modulus are assessed and selected to design compositions for high-modulus steels based on thermodynamic calculations. The steel matrix composites reinforced with boride and carbide phases are produced through ingot casting and are processed thermomechanically to strips following standard processing routes for automotive products. The results show that the Young's modulus of steels in the as-cast condition can be increased using borides and carbides. However, further down-stream processing via conventional thermomechanical processing leads to a gradual degradation of the Young's modulus due to extensive void formation. The opportunities and challenges of ceramic-reinforced high-modulus steels produced via conventional ingot casting and thermomechanical processing for the automotive market are discussed.

A carbide-free bainitic microstructure is suitable for achieving a combination of ultra high strength and high ductility. In this work, a steel containing nominally 0.34C-2Mn-1.5Si-1Cr (wt.%) was produced via industrial hot rolling and laboratory heat treatments. The austenitization (900°C, 30 min.) and austempering (300-400°C, 3 h) treatments were done in salt bath furnaces. The austempering treatments were designed to approximately simulate the coiling step, following hot rolling and run-out-table cooling, when the bainitic transformation would take place and certain amount of austenite would be stabilized due to suppression of carbide precipitation. The microstructures and various mechanical properties (tensile properties, bendability, flangeability, and room and subzero temperature impact toughness) relevant for applications were characterized. It was found that the mechanical properties were highly dependent on the stability of the retained austenite, presence of martensite in the microstructure and the size of the microstructural constituents. The highest amount of retained austenite (~ 27 wt.%) was obtained in the sample austempered at 375°C but due to lower austenite stability and coarser overall microstructure, the sample exhibited lower tensile ductility, bendability, flangeability and impact toughness. The sample austempered at 400°C also showed poor properties due to the presence of initial martensite and coarse microstructure. The best combination of mechanical properties was achieved for the samples austempered at 325-350°C with a lower amount of retained austenite but with the highest mechanical stability.

No abstract