Ultrahigh strength hot rolled microalloyed steels: microstructural aspects of development

Ultrahigh strength hot rolled microalloyed steels of yield strength 690 and 760 MPa with ferrite–bainite microstructure have been developed. Impact toughness of ∼135 J at -40°C and a ductile–brittle transition temperature of less than approximately -70°C have been obtained in steels of gauge ∼3 mm. The attractive strength–toughness combination was achieved by applying microalloying concepts and controlled rolling in an interactive manner. Alloy design aspects are qualitatively described in relation to the contributions of solute, grain size, precipitates, and microstructural constituents.