Simultaneously Improved Strength and Impact Toughness by Introducing Ultrafine Ferrite in Low-Temperature Bainitic Steel

Ultrafine ferrite with a grain size range of 0.5-2 μm was introduced into low-temperature bainitic medium-carbon high-silicon steel through partially austenitization of cold-rolled tempered troostite followed by austempering at temperatures of 10 °C above martensite starting temperature. The microstructures and mechanical properties were studied and compared with conventional low-temperature bainitic steel obtained by full austenitization of cold-rolled tempered troostite followed by austempering at a temperature of 10 °C above martensite starting temperature. The results show that introduction of ~4 vol% ultrafine ferrite into low-temperature bainite can simultaneously enhance the ultimate tensile strength, yield strength, and impact toughness to 1614 MPa, 1074 MPa, and 52 J (half-width Charpy U-notch specimen) without loss of ductility, increasing by approximately 6.5%, 12.5%, and 62.5%, respectively.