Influence of sulphur and vanadium additions on toughness of bainitic steels

Abstract

The Charpy impact toughness of bainitic steels, in the longitudinal direction, was increased by appropriate additions of sulphur and vanadium. The Charpy impact value in the longitudinal direction was highest when the sulphur content was 0.1 wt-%. A suppression of austenite grain growth by manganese sulphides was recognised when the austenitising temperature was higher than 1373 K. Furthermore, it was found that bainite packets were refined by an increase in sulphur content, because manganese sulphides existing in the austenite grains promoted bainite transformation. The main effect of the vanadium addition was probably to enhance the above effect by precipitation of vanadium carbonitrides on the manganese sulphides. Accordingly, an increase in toughness with additions of sulphur and vanadium may be attributed to bainite packet refinement resulting from the suppression of austenite grain growth and the promotion of intragranular bainite transformation by manganese sulphides.