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 similar to 135 J at -40 degreesC and a ductile-brittle transition temperature of less than approximately -70 degreesC have been obtained in steels of gauge similar to3 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.