Ultra-low niobium (ULNb) alloying design solution for commodity grade structural steels

Abstract

Conventionally the addition of micro-alloying elements such as niobium (Nb), typically ≥ 0.020w t%Nb, has been associated with high strength low alloy (HSLA) steels with lower carbon content and processed by means of Thermo-Mechanical rolling. Here it is well established, that Nb has the strongest effect in retarding static recrystallisation of austenite during inter-pass times by precipitate pinning mechanism. Due to the lower solubility of Nb in austenite at typical carbon levels associated with commodity grade structural steels (i.e., ≥0.15wt.%C), the use of Nb has been somewhat limited and widely regarded to be applied only when higher strengths, ≥355 MPa and/or low temperature impact toughness properties are specified.However, recent laboratory studies coupled with industrial trials have demonstrated an alternative solution to make commodity grade structural steels, without making any changes to existing processing routes employed. By using Ultra-Low Niobium (ULNb) additions (i.e., ≤100ppmNb) it is possible to enable cost savings in ferro-alloying expenditure coupled with a reduction in grade specific GWPe/tonne of steel produced. This paper summarises and presents these recent findings and discusses the metallurgical background towards extracting greater value from existing commodity grade structural steels through Ultra-Low additions of Nb.