Development Trends in HSLA Steels for Welded Constructions

Abstract

Costs and time savings in the fabrication of constructions by using steel grades with improved weldability and higher strength lead to an increased realisation of steel structures. But the higher strength can only be used, provided the construction is safe, by avoiding any brittle fracture as well as the propagation of existing flaws by ductile fracture. Besides a high cleanliness of the steel, a low carbon content and the maximisation of grain refinement as strengthening mechanisms are the fundamentals of modem HSLA steel production. As result of the thermomechanical processing of microalloyed steels, high strength levels with a rather low carbon content can be produced. Actually a chemical composition with less than the 0.09% C threshold value is aimed for, in order to avoid the peritectic reaction during solidification, which is responsible for microsegregations and thus a deterioration of the heat affected zone toughness. A finer effective grain size than typical for thermomechanically rolled steels with a ferritic-pearlitic microstructure is achieved with microstructures of acicular bainite or martensite. The alloy design of such steels, which can guarantee yield strength levels of 690 MPa and more, asks for an overall higher alloy content or a faster cooling rate or combinations of both. In any case, austenite processing before transformation adds to the refinement of these microstructures. The industrial verification relies on interrupted accelerated cooling or direct quenching after thermomechanical rolling. Furthermore, the toughness in the heat affected zone is improved with a microstructure of low carbon acicular bainite.