Mechanical behaviour and microstructural characteristics of high-silicon ultra-strong bainitic steels for hot rolling practice

Abstract

High-silicon (1.5–2.5 wt%) steels were designed to achieve carbide-free bainitic matrices with retained austenite through industrial hot rolling, with coiling temperatures of 310 °C and 350 °C. The resulting ultrahigh strength steels (1409–1644 MPa) were characterized through tensile testing, Charpy impact toughness, and Kahn tear tests, while microstructural analysis was performed using scanning electron microscopy and X-ray diffraction. Both yield and tensile strengths correlated strongly with bainitic matrix characteristics, including phase fraction, dislocation density, carbon content, and plate thickness. Ductility showed dependence on film-type austenite content and Md temperature, mechanical stability. The steels exhibited exceptional impact toughness meeting industrial requirements, with ductile fracture behaviour observed down to −100 °C, challenging previous findings. Crack resistance values matched or exceeded those of comparable ultrahigh strength steels. The lower coiling temperature (310 °C) produced retained austenite with higher mechanical stability, benefiting tensile properties and crack resistance, while impact toughness remained largely unaffected by austenite stability due to high strain rates.