Aluminium nitride precipitation and grain structure of continuously cast billet corners

The highly grain refined corners of continuously cast steel billets and the high levels of associated aluminium nitride precipitation were investigated. From experiments using steels grain refined by the addition of aluminium, it was found that aluminium nitride precipitated during recalescence after the temperature of the billet corners had fallen below the Ar3 temperature in the mould and secondary cooling zone. The aluminium nitride was found to persist and prevent austenite grain growth during reheating to the straightening temperature and this produced the refined grain structure and high aluminium nitride content of the cooled billet.MST/1170