Effects of co-addition of titanium and boron on microstructure of superplastic Cr-Mo steels

The effects of titanium and boron on the microstructure of a low alloyed Cr -Mo steel with 0.6 wt-%C have been investigated by comparison with a steel containing only titanium and a steel free from both titanium and boron. Eac h of the steels was subjected to thermomechanical treatment and annealed at 700 degrees C, resulting in small grains of size a few micrometres. The st eel containing both titanium and boron possessed the smallest ferrite grain s and M3C carbines of the three examined. This is attributed to a fine disp ersion of borides (TiB2) and borocarbides (Ti(C,B)) of size 10 nm in the fe rrite matrix through the pinning effect. At the grain boundaries smalt carb ide particles were present which were effective in inhibiting grain boundar y migration. The extremely fine borides and/or borocarbides were useful in suppressing intragranular deformation of ferrite grains due to precipitatio n hardening. This may have assisted in promoting grain boundary sliding, re sulting in superior superplastic elongation. MST/4270.