INFLUENCE OF SULFUR AND NIOBIUM ON HOT DUCTILITY OF AS CAST STEELS

The influence of S and Nb on the hot ductility of as cast C-Mn-Al steel has been examined. Tensile test samples were cast in situ, cooled at 100 K min-1 to test temperatures in the range 750-1000-degrees-C, and strained to failure at a strain rate of 2 x 10(-3) s-1. Both S and Nb caused the hot ductility trough to deepen and widen, an addition of 0.03% Nb giving a deterioration in ductility similar to that obtained by increasing the S content from 0.002 to 0.016%. Grain size was coarse and similar for all steels; thus, the changes in ductility could be related to changes in precipitate and inclusion particle distribution. The addition of Nb resulted in the presence of a coarse Nb(C,N) eutectic and a fine matrix and boundary precipitation of Nb(C,N). The addition of S resulted in the precipitation of coarse sulphides at the interdendritic boundaries, as well as the presence of a fine precipitation of Fe oxysulphides similar in size and distribution to the Nb(C,N) precipitation. The apparent similarities in precipitation patterns for S and Nb suggest similar mechanisms for intergranular failure. The coarser grain boundary precipitates probably encourage grain boundary sliding and void cavitation at the boundaries. The fine matrix precipitation, by increasing the strength of the matrix, concentrates the strain into the boundary region, further reducing ductility. Because precipitation patterns at the surface of continuously cast slabs and in tensile samples are different, the experimental results cannot be transferred directly to the continous casting operation. However, it can be stated that reducing the S content will improve hot ductility and reduce transverse cracking.