Steelmaking practice for Niobium containing steels

Niobium, which is widely used as a microalloying element in steel to improve mechanical properties, in some cases, can cause problems during processing. In this paper, we discuss difficulties encountered at USINOR, especially during continuous casting and also some problems studied in a more fundamental way in close cooperation with IRSID. Niobium microalloying itself does not lead to any difficulties. Problems arise when specification requirements concerning low sulphur levels and spheroidization of the sulfides are to be met, as the procedures used for this purpose can increase nitrogen content. In this case, Steelmaking and continuous casting must be carefully controlled, as high Niobium and nitrogen content may in some cases give rise to surface and internal defects. We describe in this paper a general method for choosing a steelmaking, casting and cooling strategy, based on a study of defects and their causes, and on an experimental heat transfer study of the efficiency of secondary cooling. (AU)