A solution to the poor transverse bendability of ultra-high-strength steels has been found. Yield strength > 900 MPa combined with excellent bendability (r/t < 2.5) in all bending directions has been obtained by processing strips to produce a relatively hard bainite/martensite core combined with a softer ferrite/granular bainite surface layer.
The fact that the formability, especially in bending, tends to become poorer as the strength of the steel increases. This can limit the application of such steels, for example in telescope loader crane booms. Improved understanding of how to optimize microstructure gradients in the surface layers of the steel will help in the development of ultra-high-strength steels with better strength and formability combinations.
The use of ultra-high-strength steels in weight-critical constructions is an effective way to save energy and minimize the carbon footprint of the application. Small bending radii are used to save space in steel structures and also to make constructions stiffer. The developed solutions enabling better formability and bendability of the ultra-high-strength steels opens new application fields for these steels with outstanding properties, leading to reduce of weight of application. Therefore, fuel consumption reduces or manufacturer can use higher payloads with higher reach for the telescope applications.
Antti Kaijalainen, CASR-University of Oulu