Introducing ultrafine ferrite in low-temperature bainitic steel through a novel process for simultaneously improving strength and toughness

Abstract

A novel process is proposed to introduce ultrafine ferrite in a medium-carbon high-silicon steel that possesses low-temperature bainite. This process is based on the intercritical annealing of the cold-rolled tempered troostite and subsequent austempering at a temperature of 10 °C above martensite start temperature. The acquired microstructures, as well as the mechanical properties, were thoroughly studied and compared with the sample without ultrafine ferrite. Results show that, ultrafine ferrite with a grain size in the range of 0.5–2.0 μm is prepared in the low-temperature bainitic steel. The obtained ultrafine ferrite can be mainly attributed to the recrystallization of cold-rolled tempered troostite during intercritical annealing. Moreover, the introduction of ∼4 vol% ultrafine ferrite into the matrix with low-temperature bainite can simultaneously enhance the tensile strength, yield strength, and impact energy to 1614 MPa, 1074 MPa, and 52 J (half-width Charpy U-notch specimen), respectively, without loss of ductility. The high strength is mainly due to the fine bainitic ferrite laths, while the remarkable ductility and toughness are associated with the moderate contents of ultrafine ferrite and retained austenite within the microstructure.

Keywords

• Medium-carbon steel
• Low-temperature bainite
• Ultrafine ferrite
• Mechanical property